refactor(web): extract collab-flush path from item-detail monolith (TASK-2082)

[xarmian]

What

Behavior-preserving extraction of the Yjs collaborative-snapshot flush path out of the ~4.6k-line item detail page (web/src/routes/[username]/[workspace]/[collection]/[slug]/+page.svelte) into a new focused module. Follow-on to TASK-2029 (which extracted the raw-markdown contentSaver), part of PLAN-1984.

Extracted — web/src/lib/collab/collabFlush.svelte.ts

A createCollabFlusher() factory (mirrors contentSaver.svelte.ts's pattern). The module owns: the 5s idle debounce timer, the per-item lastFlushedContent dedupe state, and the two-stage dedupe decision (editor-space short-circuit via shouldDedupeEditorSpace + storage-space baseline/lastFlushed compare). Public API:

• schedule(ctx, markdown) — arm the 5s idle flush (was scheduleCollabFlush)
• flush(ctx, markdown, keepalive) — run a flush now, returns 'flushed'|'deduped'|'failed'|'skipped' (was runCollabFlush; called directly by the rich→raw toggle)
• flushNow(ctx, keepalive) — read live editor markdown + flush (was flushCollabNow; editor teardown + beforeunload)
• cancel() / resetDedup() — replace the inline clearTimeout(collabFlushTimer) / lastFlushedContent = null sites

Deliberately LEFT in the page (injected as callbacks)

Everything Svelte-reactive / API / editor-coupled the module can't own: the api.items.flushCollabContent PATCH + all reactive bookkeeping (saveStatus, editorStore, toast, showSaved), the op-log-cursor read + post-await force_refresh check (in save), plus isRecovering (forceRefreshInFlight), serialize (markdownToWikiLinks/cleanBrokenLinks against localIndex), normalize (unescapeDocLinks), readEditorMarkdown, isActiveItem. activeCollabContext / seededProvider / the collab $effect lifecycle stay in the page — they're deeply coupled to provider construction/teardown.

Behavior-preserving

Line-for-line faithful to the original control flow. Verified invariants:

• beforeunload: collab flushNow(ctx, true) still fires ALONGSIDE the BUG-2024 raw-markdown keepalive PATCH (both intact, unchanged ordering).
• forceRefreshInFlight gating (schedule + pre/post-await), seededProvider seeding, activeCollabContext switch-on-item-change all unchanged.
• Flush debounce timing (5s) + dedup (editor-space + storage-space) unchanged; flushDedupe.ts reused as-is.
• Item-swap resets cancel the pending flush (cancel() + resetDedup() in loadData) — no cross-item bleed.

One robustness addition from Codex review: a resetGeneration guard so an in-flight resetDedup() (item-swap / out-of-band raw save) can't be re-polluted by a stale flush's deferred lastFlushedContent commit — closes a window the await config.save() split would otherwise widen, and strengthens the same-item raw-save case that was latently racy in the original.

Line-count delta

Item page: 4649 → 4558 lines (−91). Net across files: +625 / −231 (new module + its unit tests).

Tests

New web/src/lib/collab/collabFlush.test.ts (17 tests, node project): debounce/coalesce, recovery gate, editor-space + storage-space + baseline dedupe, lastFlushed recording + active-item gate, failed/skipped non-recording, flushNow read/no-op/cancel, resetDedup, and the in-flight-reset invalidation. npm run check clean (0 errors), npm run build succeeds, all 177 node tests pass. Codex review: CLEAN.

Refs TASK-2082, PLAN-1984.