decompose reconciler roadmap into 20 implementation tasks across 5 phases

Tasks follow the architecture spec phases:
- Phase 0: key field on UElement (2 tasks + review)
- Phase 1: reactive-host bridge / fiber tree (4 tasks + review)
- Phase 2: key-based children reconciliation (3 tasks + review)
- Phase 3: unmount & dispose support (4 tasks)
- Phase 4: TypeBox value optimizations (4 tasks)

Validated with taskgraph CLI: no cycles, 15 parallel generations,
3 high-risk tasks identified (signal-driven-updates, commit-mutations,
fiber-disposal).

tasks/review-reactive-host-bridge.md

@@ -0,0 +1,46 @@
+---
+id: review-reactive-host-bridge
+name: Review reactive-host bridge
+status: pending
+depends_on: [signal-driven-updates, render-re-renderable]
+created: 2026-05-18T16:22:57.199831873Z
+modified: 2026-05-18T16:22:57.199832316Z
+scope: narrow
+risk: low
+impact: phase
+level: review
+---
+
+# Description
+
+Review the reactive-host bridge implementation before proceeding to key-based children reconciliation. This checkpoint verifies that the fiber tree is correctly built during mount, signal changes correctly propagate to `prepareUpdate`/`commitUpdate`, and re-rendering works without duplicate instances.
+
+This is critical because Phase 2 (key-based reconciliation) builds on top of the fiber tree structure and the reconciliation loop. Any bugs in the bridge will compound in the more complex reconciliation logic.
+
+## Acceptance Criteria
+
+- [ ] Fiber tree structure matches the UNode tree after mount
+- [ ] Signal-driven prop updates flow correctly through `prepareUpdate` → `commitUpdate`
+- [ ] Multiple signals changing in one batch produce one reconciliation pass
+- [ ] `render()` called twice produces one instance tree, not two
+- [ ] `signalDisposers` are correctly populated for later cleanup
+- [ ] Effect commit order is top-down (parent before child)
+- [ ] Function components remain transparent in the fiber tree
+- [ ] All existing + new tests pass
+- [ ] No signal subscription leaks (verify effect disposers stored on fibers)
+- [ ] HostConfig implementations that don't implement `prepareUpdate`/`commitUpdate` still work (no-op)
+
+## References
+
+- docs/architecture/reconciler.md — Steps 1-4
+- docs/architecture/decisions/005-signal-driven-updates-over-tree-diffing.md
+- docs/architecture/reactive-layer.md
+- docs/architecture/host-config.md
+
+## Notes
+
+> To be filled on completion of review
+
+## Summary
+
+> To be filled on completion