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/unmount-implementation.md

@@ -0,0 +1,69 @@
+---
+id: unmount-implementation
+name: Implement full Root.unmount()
+status: pending
+depends_on: [fiber-disposal, reactiveroot-dispose]
+created: 2026-05-18T16:22:57.308494756Z
+modified: 2026-05-18T16:22:57.308495198Z
+scope: moderate
+risk: medium
+impact: component
+level: implementation
+---
+
+# Description
+
+Replace the stub `Root.unmount()` with a full implementation that tears down the fiber tree, removes host instances, disposes signal subscriptions, and calls host cleanup methods.
+
+Current stub:
+```typescript
+unmount() {
+  host.finalizeRoot?.(ctx);
+  host.emit?.("root.unmount", `root_${Date.now()}`, {});
+}
+```
+
+New flow:
+```
+root.unmount()
+  → disposeFiber(rootFiber, ctx)       // disposes signals, calls finalizeInstance bottom-up
+  → removeChild for each top-level child  // host removal (top-down from root, each child subtree already disposed)
+  → host.finalizeRoot(ctx)             // root-level host cleanup
+  → emit "root.unmount"
+  → clear rootFiber reference
+```
+
+Note: `disposeFiber` handles resource cleanup (signals, finalizeInstance) but NOT `host.removeChild`. We need to walk the fiber tree and call `removeChild` for host instance removal. However, since children are already disposed, we only need to remove top-level children from the root context.
+
+Open question from lifecycle.md: Should `ReactiveRoot` auto-dispose on `root.unmount()`? The architecture doc suggests decoupling is safer (a ReactiveRoot could drive multiple roots). This task should keep them decoupled — the consumer is responsible for calling `ReactiveRoot.dispose()` separately if needed.
+
+## Acceptance Criteria
+
+- [ ] `Root.unmount()` calls `disposeFiber(rootFiber, ctx)`
+- [ ] `Root.unmount()` calls `host.removeChild` for top-level children
+- [ ] `host.finalizeRoot(ctx)` called after disposal and removal
+- [ ] `host.emit("root.unmount", ...)` still fires
+- [ ] `rootFiber` reference cleared after unmount
+- [ ] Subsequent `render()` calls after `unmount()` start fresh (new mount)
+- [ ] Signal subscriptions from the fiber tree are cleaned up
+- [ ] Does NOT auto-dispose `ReactiveRoot` — consumer responsibility
+- [ ] `unmount()` is idempotent (safe to call twice)
+- [ ] Existing tests pass
+- [ ] New test: unmount cleans up all host instances
+- [ ] New test: unmount then re-render works (fresh mount)
+- [ ] New test: double unmount is safe
+- [ ] New test: unmount calls finalizeInstance for each fiber
+
+## References
+
+- docs/architecture/lifecycle.md — Root Unmount Flow, Host Notification
+- docs/architecture/host-config.md — Known Gaps: unmount is a stub
+- docs/architecture/lifecycle.md — Open Question 2 (ReactiveRoot auto-dispose on unmount)
+
+## Notes
+
+> To be filled by implementation agent
+
+## Summary
+
+> To be filled on completion