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/mount-with-fibers.md

@@ -0,0 +1,48 @@
+---
+id: mount-with-fibers
+name: Refactor mountNode to build fiber tree
+status: pending
+depends_on: [fiber-type]
+created: 2026-05-18T16:22:57.152634856Z
+modified: 2026-05-18T16:22:57.152635298Z
+scope: moderate
+risk: medium
+impact: phase
+level: implementation
+---
+
+# Description
+
+Refactor the existing `mountNode` in `src/host/config.ts` to build a fiber tree alongside host instances during the mount phase. Currently, `mountNode` creates instances but discards them — the instance tree is unreachable for updates. After this task, `Root` holds a reference to the root fiber, enabling re-render and unmount.
+
+The mount pipeline stays the same (depth-first, post-order append). The change is that each created instance is wrapped in a `Fiber` node that tracks its parent, children, and props. Function components remain transparent — they don't get their own fiber.
+
+This task also stores the root fiber on the `Root` object so that subsequent renders can diff against it.
+
+## Acceptance Criteria
+
+- [ ] `mountNode` creates `Fiber` nodes for each intrinsic element and text instance
+- [ ] Function components are transparent — no fiber for the component itself
+- [ ] `URoot` children are mounted into the parent fiber (root is a transparent container)
+- [ ] Primitives get text fibers with `tag: "#text"`
+- [ ] Fiber tree is linked: `parent` and `children` fields are populated
+- [ ] `Root` object holds a `rootFiber` reference (new field)
+- [ ] `signalDisposers` initialized as empty array on each fiber
+- [ ] `effect` initialized as `null` on each fiber
+- [ ] Existing mount behavior preserved — hosts still receive same `createInstance`/`appendChild` calls
+- [ ] Existing tests pass
+- [ ] New test: after `render()`, `Root.rootFiber` has correct children structure matching the UNode tree
+
+## References
+
+- docs/architecture/reconciler.md — Fiber Node, Reconciliation Algorithm, Function Components sections
+- docs/architecture/host-config.md — Mount Pipeline, No Instance Tree Reference gap
+- docs/architecture/lifecycle.md — Fiber Tree Disposal
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion