add reconciler architecture docs and update existing docs with cross-references

Phase 2: transitioning reconciler research into architecture documents.

New docs:
- reconciler.md: fiber tree, reconciliation algorithm (signal-driven
  props + key-based children), update scheduling, commit order,
  TypeBox optimization layer, file structure, consumer impact
- lifecycle.md: mount/update/dispose phases, fiber tree disposal,
  partial tree removal, ReactiveRoot.dispose(), finalizeInstance,
  idempotent disposal, computed vs effect cleanup
- ADR-004: key as first-class field on UElement (not a prop)
- ADR-005: signal-driven updates for props, reconciliation for
  structure (hybrid approach, not full tree diffing)

Updated docs:
- README.md: add reconciler.md, lifecycle.md, ADRs 004/005 to
  index; update reconciler roadmap with architecture doc links
- schema.md: add key?: string to UElement type with TODO comment;
  update known gaps to reference ADR-004 and reconciler.md;
  rephrase key constraint as temporary
- element-factory.md: update key extraction gap to reference
  ADR-004 and reconciler.md
- host-config.md: reference reconciler.md and lifecycle.md
  for the reconciler bridge and disposal gaps
- reactive-layer.md: reference reconciler.md and lifecycle.md
  for the signal-host bridge and disposal gaps
- events.md: reference lifecycle.md for unmount/dispose gap

docs/architecture/events.md

@@ -136,7 +136,7 @@ The event map does not include error events (e.g., `instance.error`, `render.err
 
 ### No cleanup on unmount
 
-The current `unmount()` implementation in `HostConfig` does not tear down event subscriptions. If a pubsub emitter was wired to the host's `emit()` during render, unmount does not unsubscribe. This gap is shared with the broader reconciler disposal gap — see [reactive-layer.md](reactive-layer.md) (no-op `dispose` functions) and the unmount & dispose research ([03-unmount-dispose-support.md](../../research/reconciler/03-unmount-dispose-support.md)).
+The current `unmount()` implementation in `HostConfig` does not tear down event subscriptions. If a pubsub emitter was wired to the host's `emit()` during render, unmount does not unsubscribe. This gap is shared with the broader reconciler disposal gap — see [lifecycle.md](lifecycle.md) and the unmount & dispose research (`docs/research/reconciler/03-unmount-dispose-support.md`).
 
 ## Constraints