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>

build.gradle.kts

@@ -0,0 +1,12 @@
+// Top-level build for floret-kit. Plugins are declared here (apply false) and
+// applied per-module. Every module shares the family group + version so the
+// consuming apps can depend on `de.jeanlucmakiola.floret:<module>` and Gradle's
+// composite-build substitution maps it to the local source module.
+plugins {
+    alias(libs.plugins.kotlin.jvm) apply false
+}
+
+subprojects {
+    group = "de.jeanlucmakiola.floret"
+    version = "0.1.0"
+}