soma/range-calendar: fix endpoint clear + stop auto-paging on select

Closes 2 bugs from the date-range-picker incidencia 2026-05-20:

#1 — `RangeCalendarProvider` no longer auto-shifts the placeholder when
a selection lands in the last visible month. The visible months stay
where the user put them; navigation is explicit (prev/next/month/year
controls or keyboard). `reanchorInitialSelection` is removed and its
`shift-navigate` trigger goes with it.

#2 — clicking an endpoint of a completed range now drops only that
endpoint and re-anchors on the surviving one. The previous behavior
cleared both endpoints, which forced users to rebuild the entire range
to amend it. The provider already implemented this; only the test
codified the old behavior. Test rewritten to match the documented
intent and symmetric for start/end.

The two range-calendar tests that previously asserted the wrong
behavior now cover the correct invariants.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/uix/soma/components/range-calendar/range-calendar-provider.svelte.test.ts

@@ -191,7 +191,11 @@ describe('RangeCalendarProvider', () => {
 		dom.dispose();
 	});
 
-	it('clears a completed range when an endpoint is clicked again', () => {
+	it('clears only the clicked endpoint of a completed range, keeping the other', () => {
+		// Incidencia 2026-05-20 #2 — clicking the end of a complete range must
+		// drop just that endpoint and re-anchor on the surviving start (the
+		// user is amending the range, not discarding it). The symmetric click
+		// on the start must drop only the start and keep the end.
 		const { dom } = installSomaHarness();
 		const opts = createRangeCalendarOpts();
 		const start = new CalendarDate(2026, 5, 10);
@@ -205,9 +209,17 @@ describe('RangeCalendarProvider', () => {
 		provider.select(end);
 		expect(opts.value.current).toEqual({ start, end });
 
+		// Click end → end drops, start stays, anchor returns to start.
 		provider.select(end);
-		expect(opts.value.current).toEqual({ start: undefined, end: undefined });
+		expect(opts.value.current).toEqual({ start, end: undefined });
-		expect(provider.anchor).toBeUndefined();
+		expect(provider.anchor).toEqual(start);
+
+		// Re-close the range, then click start → start drops, end stays.
+		provider.select(end);
+		expect(opts.value.current).toEqual({ start, end });
+		provider.select(start);
+		expect(opts.value.current).toEqual({ start: undefined, end });
+		expect(provider.anchor).toEqual(end);
 
 		cleanup();
 		dom.dispose();
@@ -239,7 +251,11 @@ describe('RangeCalendarProvider', () => {
 		dom.dispose();
 	});
 
-	it('advances only the initial selection made in the last visible month', () => {
+	it('never auto-pages the placeholder on selection (incidencia 2026-05-20 #1)', () => {
+		// Selection must not re-anchor the visible months. The user controls
+		// what's on screen via prev/next/month/year — clicking a date in any
+		// position (including the last visible month) keeps the popup where
+		// they put it.
 		const { dom } = installSomaHarness();
 		const opts = createRangeCalendarOpts();
 		opts.numberOfMonths.current = 2;
@@ -256,12 +272,14 @@ describe('RangeCalendarProvider', () => {
 		expect(opts.value.current).toEqual({ start: mayStart, end: juneEnd });
 		expect(opts.placeholder.current.toString()).toBe('2026-05-01');
 
+		// A new selection that lands in the last visible month (June) used to
+		// shift the popup to June+July. It must stay put.
 		provider.select(new CalendarDate(2026, 6, 20));
 		expect(opts.value.current).toEqual({
 			start: new CalendarDate(2026, 6, 20),
 			end: undefined
 		});
-		expect(opts.placeholder.current.toString()).toBe('2026-06-01');
+		expect(opts.placeholder.current.toString()).toBe('2026-05-01');
 
 		cleanup();
 		dom.dispose();

src/uix/soma/components/range-calendar/range-calendar-provider.svelte.ts

@@ -14,7 +14,6 @@ import {
 	type WeekStartsOn,
 	type DateOrder,
 	isSameDay,
-	isSameMonth,
 	startOfMonth,
 	endOfMonth,
 	getLocalTimeZone,
@@ -378,10 +377,13 @@ export class RangeCalendarProvider {
 	}
 
 	private startSelection(date: DateValue, target?: HTMLElement): void {
+		// Incidencia 2026-05-20 #1 — never auto-shift the placeholder on
+		// selection. The visible months stay where the user put them; if they
+		// need to see a different range they navigate explicitly via prev/next
+		// or month/year selects. Click-to-select must not page the popup.
 		this.anchor = date;
 		this.previewDate = undefined;
 		this.opts.value.current = { start: date, end: undefined };
-		this.reanchorInitialSelection(date);
 		void this.runtime.trigger('commit-start', target ? { fallbackTarget: target } : undefined);
 	}
 
@@ -419,17 +421,6 @@ export class RangeCalendarProvider {
 		void this.runtime.trigger('commit-clear', target ? { fallbackTarget: target } : undefined);
 	}
 
-	private reanchorInitialSelection(date: DateValue): void {
-		const count = Math.max(1, this.opts.numberOfMonths.current);
-		if (count < 2) return;
-		const visible = this.visibleMonths;
-		const last = visible[visible.length - 1];
-		if (!last || !isSameMonth(date, last)) return;
-		this.opts.placeholder.current = startOfMonth(date);
-		this.announceNavigation();
-		void this.runtime.trigger('shift-navigate');
-	}
-
 	private commit(a: DateValue, b: DateValue, target?: HTMLElement): void {
 		const [start, end] = orderPair(a, b);