Add offline body-CRDT sync E2E coverage

[h4yfans]

What

Add deterministic dual-device desktop E2E coverage for body-only note CRDT sync, and fix the sync/runtime gaps that the new coverage exposed.

Why

We needed reliable regression coverage for the core offline sync scenarios across two devices:

• offline note creation
• offline and mixed online/offline edits
• concurrent same-note merges
• reconnect-order variants
• the explicit 2 notes / 4 edits cross-edit flow

Without this, the desktop app could regress on convergence behavior without a stable end-to-end signal.

How

• Added a dual-device Electron E2E harness with isolated device identities and shared sync bootstrap.
• Added deterministic test controls for network state, manual sync, note creation/open/edit/read helpers, and CRDT/writeback inspection.
• Added body-CRDT E2E suites for create propagation, single-writer propagation, different-note concurrent edits, same-note merges, and coverage variants.
• Fixed sync issues uncovered by the new tests, including:
  • CRDT seeding for existing-but-empty docs on note open
  • immediate CRDT snapshot push after successful update batches
  • reconnect pull coverage for inactive markdown notes
  • renderer autosave racing Yjs-owned note bodies
  • re-fetching already-applied CRDT sequences after snapshot replacement
  • full-sync CRDT catch-up when the index DB is unavailable in tests
• Added and updated unit coverage for the new sync behavior.
• Added the execution checklist in docs/superpowers/plans/2026-04-10-body-crdt-sync-e2e-checklist.md.

Type

• fix — bug fix
• feat — new feature
• refactor — restructure without behavior change
• style — visual/UI only
• perf — performance improvement
• test — adding or updating tests
• chore — tooling, deps, config
• docs — documentation only
• ci — CI/CD changes

Test plan

• Unit tests added/updated
• Integration tests added/updated
• Manual testing (describe below)

Commands run:

• pnpm test
• pnpm --dir apps/sync-server exec vitest run schema/d1.test.ts
• pnpm --dir apps/desktop exec vitest run --config config/vitest.config.ts src/main/sync/engine/crdt-sync-coordinator.test.ts
• pnpm --dir apps/desktop exec vitest run --config config/vitest.config.ts src/main/sync/engine.test.ts -t "#given WS reconnect #when handleWsConnected fires"
• pnpm --dir apps/desktop exec playwright test --config config/playwright.config.ts tests/e2e/body-crdt-create-propagation.e2e.ts
• pnpm --dir apps/desktop exec playwright test --config config/playwright.config.ts tests/e2e/body-crdt-single-writer-edit.e2e.ts
• pnpm --dir apps/desktop exec playwright test --config config/playwright.config.ts tests/e2e/body-crdt-different-note-edit.e2e.ts
• pnpm --dir apps/desktop exec playwright test --config config/playwright.config.ts tests/e2e/body-crdt-same-note-merge.e2e.ts
• pnpm --dir apps/desktop exec playwright test --config config/playwright.config.ts tests/e2e/body-crdt-coverage-variants.e2e.ts
• pnpm --dir apps/desktop test:e2e -- dual-device-isolation.e2e.ts shared-sync-bootstrap.e2e.ts network-control.e2e.ts note-sync-helpers.e2e.ts manual-sync-smoke.e2e.ts body-crdt-create-propagation.e2e.ts

Checklist

• Self-reviewed the diff
• No hardcoded secrets or credentials
• Files stay under ~500 LOC
• Follows immutable data patterns