fix(timeline): track form dirtiness via change callbacks, not an $effect

The round-2 dirty-guard used an $effect that both read and wrote its `dirtyArmed`
$state, so the self-write re-triggered the effect and `dirty` flipped true on
mount — the beforeNavigate confirm then fired on every navigation away from an
untouched form (caught by the round-3 clean-agent review + the Svelte autofixer,
which flags assigning state inside $effect).

Replace it with the component's existing idiomatic pattern: DatePrecisionField,
PersonMultiSelect, and DocumentMultiSelect each gain an optional `onchange`
callback fired on a real user edit, and EventForm passes `markDirty` to all
three. Now date/precision/end-date and picker add/remove mark the form dirty
exactly like title/type/description already did — no effect, no mount-timing
trap. The new props are optional, so the other consumers (WhoWhenSection, the
document forms) are unaffected. Svelte autofixer: clean.

Addresses PR #832 review (round-3 clean-agent concern).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/shared/primitives/DatePrecisionField.svelte

@@ -36,6 +36,7 @@ let {
 	dateLabel = m.form_label_date(),
 	dateRequired = true,
 	dateError = '',
+	onchange = undefined,
 	dateTestId = undefined,
 	precisionTestId = undefined,
 	endDateInnerTestId = undefined
@@ -52,6 +53,8 @@ let {
 	dateRequired?: boolean;
 	/** Server-side date error (e.g. blank required field) wired to the field's aria-invalid. */
 	dateError?: string;
+	/** Called on any user edit (date, precision, end-date) — lets a parent track dirtiness. */
+	onchange?: () => void;
 	dateTestId?: string;
 	precisionTestId?: string;
 	endDateInnerTestId?: string;
@@ -102,12 +105,14 @@ function handleDateInput(e: Event) {
 	dateDisplay = result.display;
 	dateIso = result.iso;
 	dateDirty = true;
+	onchange?.();
 }
 
 function handleEndDateInput(e: Event) {
 	const result = handleGermanDateInput(e);
 	endDisplay = result.display;
 	endDateIso = result.iso;
+	onchange?.();
 }
 
 $effect(() => {
@@ -161,6 +166,7 @@ $effect(() => {
 		id="{dateInputName}Precision"
 		name={precisionInputName}
 		bind:value={precision}
+		onchange={() => onchange?.()}
 		class="block min-h-[48px] w-full rounded border border-line px-2 py-3 text-sm shadow-sm focus:outline-none focus-visible:ring-2 focus-visible:ring-focus-ring"
 	>
 		{#each PRECISIONS as p (p.value)}