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-reconciler.md

@@ -0,0 +1,45 @@
+---
+id: review-reconciler
+name: Review key-based reconciliation
+status: pending
+depends_on: [commit-mutations]
+created: 2026-05-18T16:22:57.261831485Z
+modified: 2026-05-18T16:22:57.261831927Z
+scope: narrow
+risk: low
+impact: phase
+level: review
+---
+
+# Description
+
+Review the full key-based reconciliation implementation before proceeding to unmount/dispose. This checkpoint verifies that the reconciler correctly handles add, remove, reorder, and update operations for both keyed and unkeyed children.
+
+This is important because Phase 3 (dispose) will add teardown logic that depends on the reconciliation algorithm correctly identifying removed fibers. If reconciliation misclassifies a child as "removed" when it should be "moved," disposal will destroy an instance that still exists.
+
+## Acceptance Criteria
+
+- [ ] Keyed children: add/remove/reorder all produce correct host calls
+- [ ] Unkeyed children: positional matching works for prop updates
+- [ ] Mixed keyed + unkeyed: key-based for keyed, positional for unkeyed
+- [ ] Duplicate keys: last-wins, no crash, warn logged
+- [ ] Commit order: removes → inserts/moves → updates
+- [ ] `insertBefore` used for moves and inserts with a target sibling
+- [ ] Fiber tree structure matches the new UNode tree after reconciliation
+- [ ] No orphaned fibers (removed fibers are pruned from parent's children)
+- [ ] No leaked host instances (every `removeChild` called for removed fibers)
+- [ ] Signal-driven prop updates still work alongside structural reconciliation
+- [ ] All tests pass including host integration tests
+
+## References
+
+- docs/architecture/reconciler.md — Full reconciliation algorithm
+- docs/architecture/lifecycle.md — Disposal depends on correct remove classification
+
+## Notes
+
+> To be filled on completion of review
+
+## Summary
+
+> To be filled on completion