fix(bulk-edit): cycle-3 polish — Felix C2/C3/C4/C5 + Sara coverage gaps

Felix C2 — `BatchMetadataRequest` controller now uses `@Valid` so future
@Size/etc. annotations on the record actually fire.

Felix C3 — Auto-clear `$effect` in `+layout.svelte` reads
`bulkSelectionStore.size` inside `untrack()` so the effect only re-fires on
route change, not on every checkbox toggle.

Felix C4 — `BulkDocumentEditLayout` edit-mode hydration loop now lives
inside `onMount` (not at top-level script) so the SvelteMap mutation is
unambiguously tied to instance lifecycle, matching the pattern used by
`WhoWhenSection`/`DescriptionSection` after the cycle-2 fix.

Felix C5 — Replaced fully-qualified `java.util.LinkedHashSet` in
`DocumentController` with a top-of-file import.

Sara coverage — six new spec files / blocks pin the cycle-1 and cycle-2
behaviours that were previously untested:
 - `WhoWhenSection.svelte.spec.ts` — onMount seeding from initialDateIso /
   initialLocation; doesn't stomp parent-bound dateIso; hideDate / editMode
   branch
 - `DescriptionSection.svelte.spec.ts` — onMount seeding from initialTitle /
   initialDocumentLocation; doesn't stomp parent-bound values; archive-box /
   archive-folder fields visible only in editMode
 - `BulkSelectionBar.svelte.spec.ts` — Esc-scope guard tests for `<dialog>`
   open and `aria-expanded` popover present
 - `BulkDocumentEditLayout.svelte.spec.ts` — topbar reads
   "Massenbearbeitung" + "werden bearbeitet" in edit mode (not the
   upload-flavoured "hochladen"/"werden erstellt" copy)
 - `DocumentControllerTest.patchBulk_returns400_whenArchiveBoxExceeds255Chars`
   — pins the @Size validator on archiveBox via the @Valid wiring

Refs #225, PR #331

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/+layout.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 import './layout.css';
 import { page } from '$app/state';
-import { onMount } from 'svelte';
+import { onMount, untrack } from 'svelte';
 import * as m from '$lib/paraglide/messages.js';
 import LanguageSwitcher from '$lib/components/LanguageSwitcher.svelte';
 import ThemeToggle from '$lib/components/ThemeToggle.svelte';
@@ -22,6 +22,9 @@ provideConfirmService();
 // surface the BulkSelectionBar. Without this the selection silently follows
 // the user to /persons / /admin etc. and reappears as a stale 3-doc count
 // when they wander back to /documents — Felix C4 on PR #331.
+//
+// `bulkSelectionStore.size` is read inside `untrack` so the effect only
+// re-fires on route change, not on every checkbox toggle (Felix C3 cycle 3).
 $effect(() => {
 	const path = page.url.pathname;
 	const inBulkContext =
@@ -29,8 +32,10 @@ $effect(() => {
 		path.startsWith('/documents/') ||
 		path === '/enrich' ||
 		path.startsWith('/enrich/');
-	if (!inBulkContext && bulkSelectionStore.size > 0) {
-		bulkSelectionStore.clear();
+	if (!inBulkContext) {
+		untrack(() => {
+			if (bulkSelectionStore.size > 0) bulkSelectionStore.clear();
+		});
 	}
 });