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>

.gitignore

@@ -0,0 +1,16 @@
+# Gradle
+.gradle/
+build/
+
+# Local configuration
+local.properties
+
+# IDE
+*.iml
+.idea/
+
+# OS
+.DS_Store
+
+# Logs
+*.log