Yjs Testing Execution

Goal

Implement the March 22 Yjs testing plan with:

[completed] Phase 1: inspect Yjs seams, repo test patterns, and script wiring
[completed] Phase 2: add fast deterministic and plugin-contract specs
[completed] Phase 3: add slow collaboration harness, fixtures, and pnpm test:slow -- packages/yjs/src
[in_progress] Phase 4: extend the slow lane with upstream-driven reconnect and ordering cases
[pending] Phase 5: run targeted tests, slow suite, build, typecheck, lint

No /react coverage.
No browser lane.
Slow Yjs collaboration tests must not run as part of bun test.
Use upstream slate-yjs and yjs for invariants, not for blind file mirroring.

tooling/scripts/test-fast.mjs only discovers *.spec.ts[x], so *.slow.ts is enough to keep the slow lane out of default discovery.
Bun ships mock.module(...), and the local bun-types docs confirm it is available in this repo.
BaseYjsPlugin is the core risk surface: provider instantiation, sync wait, seed behavior, and cleanup.
withPlateYjs is worth direct coverage because it has real branching and composition order.
Upstream slate-yjs mostly validates operation-level Slate/Yjs correctness and should not be mirrored in this wrapper package.
Upstream yjs late-sync and pending-update cases are worth adapting at the provider-sync layer, which justified the disconnected-concurrent and out-of-order slow fixtures.