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>

gradle/libs.versions.toml

@@ -0,0 +1,16 @@
+# Version catalog for floret-kit. Kept in lockstep with the consuming apps'
+# catalogs (same Kotlin + test stack) until the catalog itself is shared.
+[versions]
+kotlin = "2.3.21"
+junit = "6.1.0"
+junitPlatform = "6.1.0"
+truth = "1.4.5"
+
+[libraries]
+junit-jupiter-api = { group = "org.junit.jupiter", name = "junit-jupiter-api", version.ref = "junit" }
+junit-jupiter-engine = { group = "org.junit.jupiter", name = "junit-jupiter-engine", version.ref = "junit" }
+junit-platform-launcher = { group = "org.junit.platform", name = "junit-platform-launcher", version.ref = "junitPlatform" }
+truth = { group = "com.google.truth", name = "truth", version.ref = "truth" }
+
+[plugins]
+kotlin-jvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlin" }