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