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/commit-mutations.md

@@ -0,0 +1,56 @@
+---
+id: commit-mutations
+name: Commit insert/move/remove effects in tree order
+status: pending
+depends_on: [lis-move-detection]
+created: 2026-05-18T16:22:57.246628537Z
+modified: 2026-05-18T16:22:57.246628975Z
+scope: moderate
+risk: high
+impact: phase
+level: implementation
+---
+
+# Description
+
+Implement the commit phase that applies all pending effects (insert, move, remove, update) to the host in the correct order. This is where fiber effects become host mutations.
+
+The commit order matters for host correctness:
+1. **Removes** — reverse order (children before parents, bottom-up)
+2. **Inserts + Moves** — left-to-right using `insertBefore` or `appendChild`
+3. **Updates** — top-down (parent before child, so parent state is consistent)
+
+This task also creates new fibers for inserted children (via `mountNode`-style recursive creation) and updates the fiber tree structure after commit.
+
+`commitMutations` is called after the reconciliation algorithm has classified all changes and queued effects on fibers. It walks the fiber tree and applies effects in the specified order.
+
+## Acceptance Criteria
+
+- [ ] `commitMutations(rootFiber, ctx)` function implemented in `src/host/reconcile.ts`
+- [ ] Removes committed in reverse order (children before parents)
+- [ ] `host.removeChild(parent, child, ctx)` called for each removal
+- [ ] Inserts committed left-to-right, using `host.appendChild` or `host.insertBefore`
+- [ ] New fibers created for inserted children, linked into parent's children array
+- [ ] Moves committed left-to-right using `host.insertBefore(parent, child, before, ctx)`
+- [ ] Updates committed top-down (parent before child) via `host.commitUpdate`
+- [ ] Fiber tree is updated after commit: new children added, removed children pruned, moved children reordered
+- [ ] Order guarantee: removes → inserts/moves → updates
+- [ ] Existing tests pass
+- [ ] New test: adding a child calls `host.appendChild`
+- [ ] New test: removing a child calls `host.removeChild`
+- [ ] New test: reordering children calls `host.insertBefore` for moved children
+- [ ] New test: mixed add+remove+update operations apply in correct order
+- [ ] New test: `insertBefore` falls back to `appendChild` if `before` is null
+
+## References
+
+- docs/architecture/reconciler.md — Step 3 (commit order), Step 4 (Commit Effects), Effect Types
+- docs/architecture/host-config.md — insertBefore, removeChild, appendChild
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion