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/render-re-renderable.md

@@ -0,0 +1,48 @@
+---
+id: render-re-renderable
+name: Make Root.render() re-renderable
+status: pending
+depends_on: [mount-with-fibers]
+created: 2026-05-18T16:22:57.183371873Z
+modified: 2026-05-18T16:22:57.183372316Z
+scope: moderate
+risk: medium
+impact: component
+level: implementation
+---
+
+# Description
+
+Make `Root.render()` support being called multiple times. Currently, calling `render()` twice creates two independent instance trees appended alongside each other. After this task, the second `render()` call reconciles against the stored fiber tree: it compares the new UNode tree to the existing fiber tree and applies property-only updates (structural reconciliation is Phase 2).
+
+This task also addresses the `render()` → `mount()` naming question from host-config.md Open Question 1: consider whether renaming `render()` to `mount()` for the first call makes the semantic clearer.
+
+For Phase 1 (before key-based reconciliation), `render()` handles:
+- First call: mounts the tree and builds the fiber tree
+- Subsequent calls: reconciles props positionally (same structure assumed), calls `prepareUpdate`/`commitUpdate` for changed props
+
+## Acceptance Criteria
+
+- [ ] `Root.render(node)` on first call mounts and builds fiber tree (existing behavior)
+- [ ] `Root.render(node)` on second call reconciles props against existing fiber tree
+- [ ] Second `render()` does NOT create duplicate instances
+- [ ] Positional children matching: Nth old child → Nth new child
+- [ ] If child count differs, excess old children remain (structural changes deferred to Phase 2)
+- [ ] `Root.unmount()` still a stub (proper unmount is Phase 3)
+- [ ] Existing tests pass
+- [ ] New test: calling `render()` twice on same root produces one instance tree with updated props
+- [ ] New test: `prepareUpdate` is called for changed props on re-render
+
+## References
+
+- docs/architecture/host-config.md — Known Gaps: The Reconciler Gap, render() is not idempotent
+- docs/architecture/reconciler.md — Step 2 (Reconcile Props), Changes to Existing Files
+- docs/architecture/host-config.md — Open Question 1 (render → mount rename)
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion