core-locale: locale-aware date formatting (currentLocale, localizedDateFormatter)

Both functions lived in Calendula's ui/common/LocaleSupport.kt. Neither is
calendar-specific — "lay this date out the way this language writes dates" is
family plumbing — so they move here, where Agendula can reach them too.

localizedDateFormatter(locale, skeleton) takes a field list ("LLLLy", "dMMMy")
and lets the locale arrange it, which is the only way to render a date correctly
in a language you didn't hand-write a template for. Dropping a field from the
skeleton drops it from the output, so callers ask for "no year" by omitting it
rather than post-processing a longer pattern.

currentLocale() reads the display locale from LocalConfiguration. This module's
own AppLanguage.apply() changes the language inside a running process, so a
locale captured once goes stale; keying on the configuration makes the UI follow
the switch.

core-locale gains Compose (runtime + ui only) for currentLocale(). Its build file
previously declared itself Compose-free, but the alternative — parking the hook
in a Compose module that depends back on this one — splits one concern across two
modules to honour a comment. core-crash is the precedent for a core-* module
carrying Compose.

Leaves core-time's Instant.formatDate() family alone despite the overlap: it is
unused by Calendula but visible to Agendula, so reconciling it is that app's
call, not a drive-by here. Noted in the changelog.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -10,6 +10,38 @@ are no binary releases; "version" tracks the shared `version` in the root build
 
 ## [Unreleased]
 
+### Added
+
+- **`core-locale`** — the locale-aware formatting half of the module, so apps
+  stop hand-rolling `"$day. $month $year"` templates that bake one language's
+  field order (and its separators, and its ordinal dot) into every language they
+  ship:
+  - `localizedDateFormatter(locale, skeleton)` — a `DateTimeFormatter` for a
+    field skeleton ("LLLLy", "dMMMy", "EEEdMMM") laid out in the locale's own
+    order, via Android's best-pattern matching. Drop a field from the skeleton to
+    drop it from the output — the locale still supplies the arrangement.
+  - `currentLocale()` — the display locale, observed from `LocalConfiguration`.
+    Since `AppLanguage.apply()` changes the language inside a running process, a
+    locale captured once goes stale; this keys recomposition to the change.
+
+### Changed
+
+- **`core-locale`** now carries Compose (runtime + ui only) for `currentLocale()`.
+  The module previously declared itself Compose-free; applying a language
+  mid-process is its own API, so the hook that makes the UI notice belongs beside
+  it rather than in a Compose module depending back on this one. `core-crash` is
+  the precedent for a `core-*` module carrying Compose.
+
+### Known gaps
+
+- `core-time`'s `Instant.formatDate()` family now overlaps `core-locale`. Its
+  private `dateNoYearFormatter` strips the year by regex-ing the pattern string,
+  which `localizedDateFormatter(locale, "dMMM")` does properly; and its
+  formatters resolve `Locale.getDefault()` once at class-init ("locale changes
+  within a running process are rare enough to ignore"), which `AppLanguage.apply()`
+  makes untrue. Left alone for now — it is unused by Calendula but Agendula-visible.
+  Reconcile when Agendula next re-pins.
+
 ## [0.1.0] - 2026-07-07
 
 ### Added

README.md

@@ -36,7 +36,7 @@ See [`docs/ARCHITECTURE.md`](docs/ARCHITECTURE.md) for the full consumption mode
 |--------|------|---------------|
 | `core-time` | JVM | Date/time helpers: `DayWindow` local-day windows, locale/zone-aware `Instant` formatting, `TimeBridge` (millis ↔ `Instant`). No Android, no deps. |
 | `core-reminders` | JVM | Reminder-lead plumbing: the lead-time unit model (`ReminderUnit`, `decomposeReminderMinutes`), the per-target override model (`ReminderOverride` + `reminderLeadFor` / `applyReminderOverride`) and the stored-format codec (`ReminderOverrideCodec`, separators configurable per app). Each app layers its own DataStore + string labels on top. |
-| `core-locale` | Android | Per-app language plumbing (`AppLanguage`): read the shipped languages from the app's `res/xml/locales_config.xml`, get/set the applied language via `AppCompatDelegate`, render each language's autonym. The app passes its own `locales_config`; appcompat only, no Compose. |
+| `core-locale` | Android | Everything "what language are we in". Per-app language plumbing (`AppLanguage`): read the shipped languages from the app's `res/xml/locales_config.xml`, get/set the applied language via `AppCompatDelegate`, render each language's autonym. Plus `currentLocale()` (observes the applied language from Compose) and `localizedDateFormatter(locale, skeleton)` (lays a field skeleton out in the locale's own order). The app passes its own `locales_config`. |
 | `core-crash` | Android | Privacy-respecting on-device crash capture + report dialog + issue-tracker hand-off. Parameterized per app via `CrashConfig`. |
 | `identity` | Android | The M3 Expressive theme factory `FloretExpressiveTheme(…)`, `rememberNavSlideSpec()`, the content transitions (`expandEnter`/`collapseExit`/`itemEnter`/…) and the `predictiveBack` peek. Each app supplies its own seed/palette. |
 | `components` | Android | Shared Compose vocabulary + recipes: `GroupedSurface`/`GroupedRow`, `InlineTextField`, `OptionCard`, `CollapsingScaffold`, `OptionPicker`, `ReorderableColumn`, `DebugRibbon`, `pastelize()`, `DialogControls`, `OnboardingScaffold`, `OptionalFormSection`, `AboutCard`, `LanguagePickerRow`. |

core-locale/build.gradle.kts

@@ -1,10 +1,20 @@
-// core-locale — per-app language plumbing shared across the family: read the
-// shipped languages from res/xml/locales_config.xml, get/set the applied
-// language via AppCompatDelegate, and render each language's autonym. App-
-// agnostic — the consuming app passes its own locales_config resource id; no
-// Compose, no DataStore.
+// core-locale — everything "what language are we in", shared across the family:
+// read the shipped languages from res/xml/locales_config.xml, get/set the applied
+// language via AppCompatDelegate, render each language's autonym, observe the
+// current locale from Compose, and format dates in that locale's own field order.
+// App-agnostic — the consuming app passes its own locales_config resource id; no
+// DataStore.
+//
+// Compose is here for currentLocale() alone: applying a language mid-process is
+// this module's own API, so the hook that makes the UI notice belongs beside it
+// rather than in a Compose module that would have to depend back on this one.
+// (core-crash is the precedent for a core-* module carrying Compose.) Deliberately
+// lean: runtime + ui only, no material3, no foundation.
 plugins {
+    // AGP 9.x provides built-in Kotlin compilation, so only the Compose plugin is
+    // applied alongside the Android library plugin.
     alias(libs.plugins.android.library)
+    alias(libs.plugins.kotlin.compose)
 }
 
 android {
@@ -20,6 +30,10 @@ android {
         targetCompatibility = JavaVersion.VERSION_17
     }
 
+    buildFeatures {
+        compose = true
+    }
+
     testOptions {
         unitTests {
             all { it.useJUnitPlatform() }
@@ -39,6 +53,16 @@ dependencies {
     // annotation (XmlRes) transitives.
     implementation(libs.androidx.appcompat)
 
+    // ConfigurationCompat for currentLocale(). Reachable transitively via
+    // appcompat, but declared because this module uses it directly.
+    implementation(libs.androidx.core.ktx)
+
+    // currentLocale() only needs the runtime (@Composable, remember) and ui
+    // (LocalConfiguration) — api() so consumers can use the returned Locale in
+    // their own composables without re-declaring Compose themselves.
+    api(platform(libs.androidx.compose.bom))
+    implementation(libs.androidx.ui)
+
     testImplementation(libs.junit.jupiter.api)
     testRuntimeOnly(libs.junit.jupiter.engine)
     testRuntimeOnly(libs.junit.platform.launcher)

core-locale/src/main/kotlin/de/jeanlucmakiola/floret/locale/DateFormatting.kt

@@ -0,0 +1,33 @@
+package de.jeanlucmakiola.floret.locale
+
+import java.time.format.DateTimeFormatter
+import java.util.Locale
+
+/**
+ * A [DateTimeFormatter] for [skeleton]'s fields laid out in [locale]'s own order,
+ * via Android's best-pattern matching.
+ *
+ * A skeleton lists the *fields* you want, not their arrangement: "dMMMy" asks for
+ * day, abbreviated month and year, and the locale decides that en-US renders
+ * "Jul 17, 2026" while de-DE renders "17. Juli 2026". Hand-rolled templates like
+ * `"$day. $month $year"` can't do this — they bake one locale's order (and its
+ * separators, and its ordinal dot) into every language the app ships.
+ *
+ * Drop a field to drop it from the output: "dMMM" is the same date without the
+ * year, still in the locale's order. Prefer that over post-processing a longer
+ * pattern, which cannot know which separators belonged to the removed field.
+ *
+ * Common skeletons: "LLLLy" (standalone month + year), "dMMMy" (day, month, year),
+ * "EEEdMMM" (weekday too). [locale] should come from [currentLocale] in Compose,
+ * so formatting follows an in-process language change.
+ *
+ * NB: this delegates to `android.text.format.DateFormat`, so it needs a real
+ * Android runtime — under unit tests with `isReturnDefaultValues` it yields no
+ * pattern. Test the skeleton *choice* in your own code; the layout itself is ICU's
+ * job and is not worth mocking.
+ */
+fun localizedDateFormatter(locale: Locale, skeleton: String): DateTimeFormatter =
+    DateTimeFormatter.ofPattern(
+        android.text.format.DateFormat.getBestDateTimePattern(locale, skeleton),
+        locale,
+    )

core-locale/src/main/kotlin/de/jeanlucmakiola/floret/locale/LocaleCompose.kt

@@ -0,0 +1,26 @@
+package de.jeanlucmakiola.floret.locale
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.platform.LocalConfiguration
+import androidx.core.os.ConfigurationCompat
+import java.util.Locale
+
+/**
+ * The current display [Locale], read observably from [LocalConfiguration].
+ *
+ * Use this rather than [Locale.getDefault] anywhere a composable formats a date,
+ * a name or a number. [AppLanguage.apply] changes the language inside a running
+ * process, so a locale captured once — in a `val`, a top-level formatter, or a
+ * `remember {}` with no key — goes stale and keeps rendering the old language.
+ * Reading through [LocalConfiguration] keys recomposition to the configuration,
+ * so the UI follows the switch. (Android lint flags the alternative as
+ * NonObservableLocale.)
+ */
+@Composable
+fun currentLocale(): Locale {
+    val configuration = LocalConfiguration.current
+    return remember(configuration) {
+        ConfigurationCompat.getLocales(configuration).get(0) ?: Locale.getDefault()
+    }
+}