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/key-matching-algorithm.md

@@ -0,0 +1,55 @@
+---
+id: key-matching-algorithm
+name: Key-based old↔new child matching
+status: pending
+depends_on: [review-reactive-host-bridge]
+created: 2026-05-18T16:22:57.216123897Z
+modified: 2026-05-18T16:22:57.216124341Z
+scope: narrow
+risk: medium
+impact: component
+level: implementation
+---
+
+# Description
+
+Implement the key-based matching algorithm that maps old children to new children by `key` field. When the tree structure changes (children added, removed, reordered), positional matching is insufficient — it would destroy and recreate instances instead of moving them.
+
+This algorithm builds two maps from the old and new child lists:
+- `oldKeyMap`: `Map<key | null, Fiber>` — old children keyed by their `key` field (positional fallback for `null` keys)
+- New children are classified as: matched (key exists in old), added (new key), removed (old key not in new)
+
+For matched children:
+- Same type: will reconcile props (same as Step 2)
+- Type changed: remove old + insert new
+
+Unkeyed children (key = undefined) fall back to positional matching.
+
+## Acceptance Criteria
+
+- [ ] `reconcileChildren` function implemented in `src/host/reconcile.ts`
+- [ ] Builds `oldKeyMap` from existing fiber children
+- [ ] Classifies new children into matched, added, removed sets
+- [ ] Matched children with same type → flag for prop reconciliation
+- [ ] Matched children with different type → old removed, new inserted
+- [ ] Unkeyed children (key = undefined) use positional matching fallback
+- [ ] Duplicate keys: warn and use last-wins (per ADR-004 consequences)
+- [ ] Pure function — no side effects, returns classification for downstream use
+- [ ] New test: keyed children reordered → matched correctly by key
+- [ ] New test: keyed child added at start → old children matched, new child added
+- [ ] New test: keyed child removed → classified as removed
+- [ ] New test: mixed keyed and unkeyed children → key-based for keyed, positional for unkeyed
+- [ ] New test: duplicate key → last-wins, no crash
+
+## References
+
+- docs/architecture/reconciler.md — Step 3 (Reconcile Children, Key-Based)
+- docs/architecture/decisions/004-key-as-first-class-field.md — ADR-004, duplicate key behavior
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion