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-key-field.md

@@ -0,0 +1,42 @@
+---
+id: review-key-field
+name: Review key field implementation
+status: pending
+depends_on: [key-on-uelement, key-extraction-in-h]
+created: 2026-05-18T16:22:57.125725833Z
+modified: 2026-05-18T16:22:57.125726289Z
+scope: narrow
+risk: low
+impact: phase
+level: review
+---
+
+# Description
+
+Review the implementation of the `key` field on `UElement` before proceeding to the fiber tree and reconciler phases. This is a critical checkpoint because every downstream task depends on `key` being correctly added to the schema and extracted by `h()`.
+
+Verify schema/type consistency, backward compatibility, and that ADR-004's consequences are properly handled.
+
+## Acceptance Criteria
+
+- [ ] `UElement` TypeScript type matches TypeBox Module schema for `key` field
+- [ ] `key` never appears in component function props
+- [ ] Backward compatibility: unkeyed elements work identically to pre-key behavior
+- [ ] TypeBox `Value.Check` passes for both keyed and unkeyed elements
+- [ ] No unintended `key: undefined` serialization on unkeyed elements (verify JSON output)
+- [ ] All existing + new tests pass
+- [ ] `isUElement` guard still correctly discriminates from `URoot`
+
+## References
+
+- docs/architecture/schema.md
+- docs/architecture/decisions/004-key-as-first-class-field.md
+- docs/architecture/element-factory.md
+
+## Notes
+
+> To be filled on completion of review
+
+## Summary
+
+> To be filled on completion