core-time: add LocalDate.isoWeekNumber() (ISO-8601 week-of-year)

Centralises the calendar-week-number computation — ISO-8601
week-of-week-based-year (Monday-start, week 1 holds the first Thursday) —
that Calendula previously inlined identically in its Week and Month grid
headers. Names the scheme choice (there are several incompatible "week
number" definitions) and covers the year-boundary rollover cases with a
unit test.

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

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

@@ -0,0 +1,16 @@
+package de.jeanlucmakiola.floret.time
+
+import java.time.LocalDate
+import java.time.temporal.IsoFields
+
+/**
+ * ISO-8601 week-of-week-based-year (1..53) for [this] date: weeks start on
+ * Monday and week 1 is the one containing the year's first Thursday. This is the
+ * calendar-week number shown in week/month grids.
+ *
+ * Centralised here so every surface agrees on one scheme — "week number" has
+ * several incompatible definitions (ISO vs. locale-first-day vs. US), and the
+ * ISO year a date belongs to can differ from its calendar year around New Year
+ * (e.g. 2018-12-31 is week 1 of 2019; 2021-01-01 is week 53 of 2020).
+ */
+fun LocalDate.isoWeekNumber(): Int = get(IsoFields.WEEK_OF_WEEK_BASED_YEAR)

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

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+package de.jeanlucmakiola.floret.time
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.Test
+import java.time.LocalDate
+
+class IsoWeekTest {
+
+    @Test
+    fun `first Thursday's week is week 1`() {
+        // 2026-01-01 is a Thursday → ISO week 1.
+        assertThat(LocalDate.of(2026, 1, 1).isoWeekNumber()).isEqualTo(1)
+    }
+
+    @Test
+    fun `mid-year week`() {
+        assertThat(LocalDate.of(2026, 6, 15).isoWeekNumber()).isEqualTo(25)
+    }
+
+    @Test
+    fun `late-December days can roll into week 1 of the next ISO year`() {
+        // 2018-12-31 (Mon) belongs to ISO week 1 of 2019.
+        assertThat(LocalDate.of(2018, 12, 31).isoWeekNumber()).isEqualTo(1)
+    }
+
+    @Test
+    fun `early-January days can belong to the last week of the previous ISO year`() {
+        // 2021-01-01 (Fri) belongs to ISO week 53 of 2020.
+        assertThat(LocalDate.of(2021, 1, 1).isoWeekNumber()).isEqualTo(53)
+    }
+
+    @Test
+    fun `a 53-week year reports week 53`() {
+        // 2020-12-31 (Thu) is ISO week 53.
+        assertThat(LocalDate.of(2020, 12, 31).isoWeekNumber()).isEqualTo(53)
+    }
+}