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/host-finalize-instance.md

@@ -0,0 +1,42 @@
+---
+id: host-finalize-instance
+name: Add finalizeInstance to HostConfig
+status: pending
+depends_on: [fiber-disposal]
+created: 2026-05-18T16:22:57.323634873Z
+modified: 2026-05-18T16:22:57.323635316Z
+scope: single
+risk: low
+impact: component
+level: implementation
+---
+
+# Description
+
+Add the optional `finalizeInstance?(instance: Instance, ctx: RootCtx): void` method to the `HostConfig` interface. This method allows hosts to perform per-instance cleanup when an instance is removed (e.g., releasing GPU buffer slots, closing database connections, removing graphology nodes).
+
+This is a backward-compatible change: the method is optional, so existing `HostConfig` implementations continue to work without modification. The reconciler calls `finalizeInstance` during `disposeFiber` after children are disposed but before the fiber's own state is cleared.
+
+## Acceptance Criteria
+
+- [ ] `finalizeInstance?(instance: Instance, ctx: RootCtx): void` added to `HostConfig` interface
+- [ ] Method is optional (existing hosts don't need to implement it)
+- [ ] Called by `disposeFiber` during fiber tree disposal
+- [ ] Called bottom-up: children's `finalizeInstance` before parent's
+- [ ] Updated in `src/host/config.ts` interface definition
+- [ ] TypeScript compiles (`npm run build:tsc`)
+- [ ] Existing tests pass (existing HostConfig implementations don't break)
+- [ ] New test: host implementing `finalizeInstance` receives calls during disposal
+
+## References
+
+- docs/architecture/lifecycle.md — Host Notification section, finalizeInstance interface
+- docs/architecture/host-config.md — Open Question 3 (should HostConfig include finalizeInstance)
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion