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/fiber-type.md

@@ -0,0 +1,40 @@
+---
+id: fiber-type
+name: Define Fiber and Effect types
+status: pending
+depends_on: [review-key-field]
+created: 2026-05-18T16:22:57.140011417Z
+modified: 2026-05-18T16:22:57.140011873Z
+scope: narrow
+risk: medium
+impact: phase
+level: implementation
+---
+
+# Description
+
+Define the `Fiber<I>` interface and `Effect` type in a new file `src/host/fiber.ts`. The fiber tree is the reconciler's internal state — it maps `UElement` positions to host instances across renders. This is the foundational type that all reconciler work builds on.
+
+The `Fiber` type is generic over `I` (matching `HostConfig`'s `Instance` type) so each fiber carries a reference to its host instance. This is the bridge: fiber → host instance → host-specific state.
+
+## Acceptance Criteria
+
+- [ ] `Fiber<I>` interface defined with all fields: `instance`, `tag`, `props`, `key`, `children`, `parent`, `effect`, `signalDisposers`, `prevProps`
+- [ ] `Effect` union type defined with variants: `update`, `insert`, `move`, `remove`
+- [ ] Types are exported from `src/host/fiber.ts`
+- [ ] Types are re-exported from `src/mod.ts` barrel
+- [ ] TypeScript compiles (`npm run build:tsc`)
+- [ ] No runtime behavior changes — this is type-only
+
+## References
+
+- docs/architecture/reconciler.md — Fiber Node, Effect Types sections
+- docs/architecture/lifecycle.md — Fiber Tree Disposal (signalDisposers field)
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion