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/key-extraction-in-h.md

@@ -0,0 +1,44 @@
+---
+id: key-extraction-in-h
+name: Extract key from props in h()
+status: pending
+depends_on: [key-on-uelement]
+created: 2026-05-18T16:22:57.111275411Z
+modified: 2026-05-18T16:22:57.111275823Z
+scope: narrow
+risk: low
+impact: component
+level: implementation
+---
+
+# Description
+
+Modify `h()` and `createRoot()` to extract `key` from props and promote it to the element-level `key` field. This ensures component functions never receive `key` in their props — it is stripped during construction, not during component invocation.
+
+Per ADR-004: `key` is a reconciler concern, not a prop. The factory layer is where the extraction happens, so that every downstream consumer (components, hosts, transforms) sees `key` only on the element, never in `props`.
+
+## Acceptance Criteria
+
+- [ ] `h()` extracts `key` from `props` before constructing the element
+- [ ] `key` is promoted to the returned element's `key` field (top-level)
+- [ ] `key` is removed from `resolvedProps` so component functions never see it
+- [ ] `createRoot()` does NOT extract `key` (URoot has no key field)
+- [ ] `h("root", { key: "x" })` — `key` stays in props for URoot (no promotion)
+- [ ] Existing tests pass (`npm run test`)
+- [ ] New test: `h("div", { key: "a" })` produces `UElement` with `key: "a"` and no `key` in `props`
+- [ ] New test: `h("div", { key: "b", class: "x" })` — `key` promoted, `class` remains in props
+- [ ] New test: `h("div", null)` — `key` is `undefined`, no `key` in props
+
+## References
+
+- docs/architecture/element-factory.md — Known Gaps: `key` prop not extracted
+- docs/architecture/decisions/004-key-as-first-class-field.md — ADR-004 decision and consequences
+- docs/architecture/reconciler.md — Fiber Node `key` field
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion