core-time: shared date/time helpers (DayWindow + Instant formatting)

First module of floret-kit, the shared Material 3 Expressive design system +
plumbing for the Floret app family. core-time is a pure-Kotlin JVM library
(DayWindow local-day boundaries + locale/zone-aware Instant formatting),
extracted from Agendula. Standalone build + 2 unit tests green.

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

README.md

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+# floret-kit
+
+The shared **Material 3 Expressive design system + plumbing** for the Floret
+family of Android apps ([Calendula](https://gitea.jeanlucmakiola.de/makiolaj/calendula),
+[Agendula](https://gitea.jeanlucmakiola.de/makiolaj/agendula), and future siblings).
+
+"Floret" is the family's design language: a Calendula flower head is a cluster of
+many small *florets*, and each app is one of them. This repo is the bloom they
+share — so a new app doesn't reinvent a settings screen, an edit form, or a theme;
+it draws from here.
+
+## How it's consumed
+
+Each app embeds this repo as a **git submodule** and wires it in with a Gradle
+**composite build**, so the kit is always built **from source** (no published
+artifacts — keeps every app reproducible for F-Droid):
+
+```kotlin
+// app's settings.gradle.kts
+includeBuild("floret-kit")
+```
+```kotlin
+// app module's build.gradle.kts
+implementation("de.jeanlucmakiola.floret:core-time")
+```
+
+Gradle's dependency substitution maps `de.jeanlucmakiola.floret:<module>` to the
+local source module — edit the kit and the app picks it up immediately.
+
+## Modules
+
+| Module | What it holds |
+|--------|---------------|
+| `core-time` | Pure-Kotlin date/time helpers: local-day windows (`DayWindow`) for smart-list logic and locale/zone-aware display formatting (`Instant.formatDate()` / `formatTime()` / `formatDateTime()`). No Android, no deps. |
+
+_More modules (identity/theme, components, screen recipes, provider/prefs/reminders/crash plumbing) land as they're extracted from the apps._
+
+## Build
+
+```sh
+./gradlew :core-time:test
+```