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>

core-time/build.gradle.kts

@@ -0,0 +1,29 @@
+// core-time — pure-Kotlin date/time helpers shared across the family:
+// local-day windows for "smart list" logic and locale/zone-aware display
+// formatting. No Android, no third-party deps — just kotlin.time + java.time
+// (available natively on minSdk 26+), so it's a plain JVM library.
+plugins {
+    alias(libs.plugins.kotlin.jvm)
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_17
+    targetCompatibility = JavaVersion.VERSION_17
+}
+
+kotlin {
+    compilerOptions {
+        jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_17
+    }
+}
+
+dependencies {
+    testImplementation(libs.junit.jupiter.api)
+    testRuntimeOnly(libs.junit.jupiter.engine)
+    testRuntimeOnly(libs.junit.platform.launcher)
+    testImplementation(libs.truth)
+}
+
+tasks.test {
+    useJUnitPlatform()
+}