date-field: opt out of value→placeholder sync in range pickers

Root cause of date-range-picker incidencia 2026-05-20 #1 (auto-paging
to next month when the popup opens with a complete range): both
endpoint inputs of a DateRangeField create their own DateFieldProvider
sharing a single placeholder. Each provider has a `$effect` that
mirrors value → placeholder so the calendar jumps to the value's
month. With two endpoints, the END field's effect overrides the
START's on every render and the popup auto-pages to the end's month.

Fix: add an explicit `syncPlaceholderToValue?: boolean` opt on
`DateFieldOpts` (default `true` — preserves single-field UX). The
DateRangeField endpoint Input passes `false`; range placeholder
coordination stays with the range provider.

Verified in browser: value `{ start: 2026-05-31, end: 2026-06-09 }`
with `placeholder = 2026-05-31` now keeps the calendar on
"May 2026 – June 2026" instead of jumping to "June – July". Both
endpoints render correctly with the start/end stripes.

The earlier `reanchorInitialSelection` removal handled the same
symptom inside the range-calendar provider for fresh selections;
this commit handles the OTHER source — the field provider auto-sync
on already-set values.

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

src/uix/soma/components/date-field/date-field-provider.svelte.ts

@@ -168,7 +168,21 @@ export interface DateFieldOpts
 			hideTimeZone: boolean;
 			required: boolean;
 			errorMessageId: string | undefined;
-		}> {}
+		}> {
+	/**
+	 * When `true` (default) the field mirrors any `value` change back into
+	 * `placeholder` so the calendar (or any sibling that consumes
+	 * `placeholder`) jumps to the month of the entered value. Standalone
+	 * date fields and single-date pickers want this.
+	 *
+	 * Range pickers MUST pass `false` for their two endpoint fields, because
+	 * both fields share a single `placeholder` and would fight: the END
+	 * field would override the START field on every render, auto-paging the
+	 * popup to the END's month (incidencia 2026-05-20 #1). Range placeholder
+	 * coordination lives in the range provider.
+	 */
+	syncPlaceholderToValue?: boolean;
+}
 
 export class DateFieldProvider {
 	readonly opts: DateFieldOpts;
@@ -266,13 +280,20 @@ export class DateFieldProvider {
 					target: opts.ref?.current
 				});
 			}
-			const placeholder = untrack(() => this.opts.placeholder.current);
-			if (this.opts.value.current && placeholder !== this.opts.value.current) {
-				untrack(() => {
-					if (this.opts.value.current) {
-						this.opts.placeholder.current = this.opts.value.current;
-					}
-				});
+			// Mirror value → placeholder for single-field cases. Range pickers
+			// opt out via `syncPlaceholderToValue: false` (incidencia 2026-05-20
+			// #1) because their two endpoint fields share one placeholder and
+			// would fight: the end input would auto-page the popup to its own
+			// month on every render.
+			if (this.opts.syncPlaceholderToValue !== false) {
+				const placeholder = untrack(() => this.opts.placeholder.current);
+				if (this.opts.value.current && placeholder !== this.opts.value.current) {
+					untrack(() => {
+						if (this.opts.value.current) {
+							this.opts.placeholder.current = this.opts.value.current;
+						}
+					});
+				}
 			}
 		});
 

src/uix/soma/components/date-range-field/components/date-range-field-input.svelte

@@ -59,6 +59,11 @@
 			}
 		),
 		placeholder: provider.opts.placeholder,
+		// Range endpoints share a single placeholder owned by the range
+		// provider; do NOT let each endpoint mirror its own value back into
+		// placeholder. Otherwise the end input continuously overrides the
+		// start input's anchor and the popup auto-pages to the end month.
+		syncPlaceholderToValue: false,
 		validate: readableActive(() => undefined), // Range-level validation lives on the parent
 		onInvalid: readableActive(() => undefined),
 		minValue: provider.opts.minValue,