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/value-clone-prevprops.md

@@ -0,0 +1,43 @@
+---
+id: value-clone-prevprops
+name: Value.Clone for prevProps snapshots
+status: pending
+depends_on: [value-equal-bailout]
+created: 2026-05-18T16:22:57.354731873Z
+modified: 2026-05-18T16:22:57.354732316Z
+scope: narrow
+risk: low
+impact: component
+level: implementation
+---
+
+# Description
+
+Use `Value.Clone` to create proper `prevProps` snapshots before mutation. Currently the architecture spec says `fiber.prevProps` should hold the snapshot before reconciliation, but without deep cloning, `prevProps` and `fiber.props` could share nested references.
+
+`Value.Clone` provides a deep clone that the `commitUpdate` contract relies on: `host.commitUpdate(instance, payload, tag, prevProps, nextProps, ctx)` needs both the before and after state to be independent.
+
+This is the second optimization in the strategy (after `Value.Equal`), and it's about correctness, not just performance. Without `Value.Clone`, hosts that read `prevProps` during `commitUpdate` might see already-mutated values.
+
+## Acceptance Criteria
+
+- [ ] `Value.Clone` imported from `@alkdev/typebox/value`
+- [ ] `fiber.prevProps` set to `Value.Clone(fiber.props)` before reconciliation updates props
+- [ ] `prevProps` is a deep clone — no shared references with `fiber.props`
+- [ ] `commitUpdate` receives independent `prevProps` and `nextProps`
+- [ ] Existing tests pass
+- [ ] New test: mutating `fiber.props` after setting `prevProps` does not affect `prevProps`
+- [ ] New test: `commitUpdate` receives correct before/after prop values
+
+## References
+
+- docs/architecture/reconciler.md — TypeBox Optimization Layer, Value.Clone row
+- docs/architecture/reconciler.md — Optimization Strategy (step 3)
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion