feat(editor): y-prosemirror binding + markdown sidecar (PRSM-56)

[nchopra]

Summary

• Replaces the lossy Y.Text("content")-as-JSON-string sync with TipTap's Collaboration extension bound to a real Y.Doc via y-prosemirror. Editor now has a true CRDT binding instead of a per-keystroke JSON re-serialize-and-stuff.
• Repurposes notes.content as a one-way debounced markdown sidecar (1.5s after last edit) — used for export, search, RAG. The authoritative state lives in yjs_updates.
• Fixes a long-standing race in compaction: compactUpToId(noteId, maxId, encoded) deletes only updates at or below a high-watermark id, so a renderer save concurrent with a compaction is never destroyed. Includes a regression test that demonstrates the original replaceYjsUpdates was lossy.
• Adds snapshot scaffolding: note_snapshots table (self-contained encoded Y.Doc state, independent of yjs_updates compaction), saveNoteSnapshot, and pruneNoteSnapshots with two-axis retention (age + count, manual snapshots exempt). No capture sites wired — PRSM-58 builds the UI on top.

What's NOT in this PR

• Hocuspocus / multi-device sync — Phase 2.
• Snapshot UI / capture sites — PRSM-58 (the scaffolding here is purely the data layer + helpers).
• Existing yXmlFragmentToProsemirrorJSON usage in collect-input.ts is @deprecated in y-prosemirror 1.3.7. Functional today; cleanup follow-up: migrate to yXmlFragmentToProseMirrorRootNode(fragment, schema).toJSON().
• COLLAB_FRAGMENT_NAME constant currently lives in markdown-to-ydoc.ts; a more neutral home (editor-constants.ts) is a follow-up.

Architectural notes for reviewers

• db/notes.ts YJS + snapshot functions now take db: DB as first parameter, matching the pre-existing pattern in db/artifacts.ts and db/skills.ts. Avoids a singleton in tests.
• NoteSyncProvider keeps a 150ms batched persister (Y.mergeUpdates) and a 1.5s markdown debouncer. destroy() flushes BOTH before setting destroyed = true (load-bearing — both flush methods short-circuit on the guard).
• Collaboration.configure({ document, field: COLLAB_FRAGMENT_NAME }) and markdownToYDocUpdate both reference the same "default" fragment constant. StarterKit undoRedo is disabled only when a Y.Doc is provided — non-collab callers (artifact editor) keep default undo.
• Compaction triggers opportunistically on every save when row count > 100. Bounded by the high-watermark, so concurrent writes survive.
• note_snapshots rows store the full encoded Y.Doc state (not a marker on yjs_updates), so aggressive compaction never invalidates a historical snapshot. Retention: 30 days / 50 newest per note, manuals never pruned.

Migrations included

• 0011_overrated_bushwacker.sql — creates note_snapshots table + composite index on (note_id, created_at). Drizzle regenerated an unrelated artifacts index in the same generation pass; that duplicate was hand-removed so the migration applies cleanly.

Test plan

• Unit: 343 vitest tests pass (pnpm --filter @prismical/desktop test). 18 new tests added across the 13 commits (compaction race regression, save-failure retry, markdown helper, sync-provider batching/debounce/destroy, artifact serializer regression, snapshot save round-trip, snapshot-independence-from-compaction, retention policies).
• pnpm --filter @prismical/desktop type:check clean.
• Manual smoke: dev DB wiped, app restarted, fresh note typed/closed/reopened — content survives. Onboarding seed loads with bullet content. Markdown sidecar converges within ~1.5s.
• DB integrity verified post-edit: 29 yjs_updates rows decoded via Y.applyUpdate → Y.XmlFragment("default") → yXmlFragmentToProsemirrorJSON produced JSON whose top-level shape (heading, paragraphs, artifact node with full attrs) matched the markdown sidecar.
• Reviewer to spot-check: open a note with an artifact, run a skill, confirm the freshest (post-keystroke) note state is fed to the skill — proves Task 9's Yjs-replay path.
• Reviewer to spot-check: ⌘Z works after editing — proves Collaboration's Y.UndoManager is wired correctly (StarterKit's undo is disabled).
• Pre-merge: wipe dev DB again on review machine — old rows from before this branch are stale Y.Text JSON and will not decode under y-prosemirror. Repo memory says "delete prismical.db is a valid fix" pre-launch.

Linked

• Spec: locally — docs/superpowers/specs/2026-05-14-prsm-56-yjs-pivot-design.md
• Plan: locally — docs/superpowers/plans/2026-05-14-prsm-56-yjs-pivot.md
• Follow-up: PRSM-58 (snapshot UI on top of the scaffolding shipped in this PR)