core-time: add TimeBridge (Instant <-> epoch millis)

Generic content-provider time bridge shared by the family: Long epoch millis
<-> kotlin.time.Instant. Extracted from Calendula for Phase 1 (Calendula as the
second core-time consumer). 3 unit tests; module suite now 5 green.

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

core-time/src/main/kotlin/de/jeanlucmakiola/floret/time/TimeBridge.kt

@@ -0,0 +1,9 @@
+package de.jeanlucmakiola.floret.time
+
+import kotlin.time.Instant
+
+/** Epoch milliseconds (as stored by Android content providers) → [Instant]. */
+fun Long.toKotlinInstantFromEpochMillis(): Instant = Instant.fromEpochMilliseconds(this)
+
+/** [Instant] → epoch milliseconds, for writing back to a content provider. */
+fun Instant.toEpochMillis(): Long = toEpochMilliseconds()

core-time/src/test/kotlin/de/jeanlucmakiola/floret/time/TimeBridgeTest.kt

@@ -0,0 +1,26 @@
+package de.jeanlucmakiola.floret.time
+
+import com.google.common.truth.Truth.assertThat
+import kotlin.time.Instant
+import org.junit.jupiter.api.Test
+
+class TimeBridgeTest {
+
+    @Test
+    fun `epoch millis round-trips through Instant`() {
+        val original = 1_717_840_800_000L // 2024-06-08T10:00:00Z
+        val instant = original.toKotlinInstantFromEpochMillis()
+        assertThat(instant.toEpochMillis()).isEqualTo(original)
+    }
+
+    @Test
+    fun `zero millis maps to Instant epoch`() {
+        assertThat(0L.toKotlinInstantFromEpochMillis()).isEqualTo(Instant.fromEpochMilliseconds(0L))
+    }
+
+    @Test
+    fun `negative epoch millis is supported`() {
+        val original = -1_000_000L
+        assertThat(original.toKotlinInstantFromEpochMillis().toEpochMillis()).isEqualTo(original)
+    }
+}