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/README.md

@@ -34,12 +34,12 @@ UJSX is functional but incomplete. The core primitives exist and are tested:
 - **Events** — `PubSubLike` and `EventEnvelope` for decoupled event emission
 - **Pointers** — `ValuePointer`, `selectNode`, `setNode` for tree navigation and targeted mutation
 
-**Known gaps** (to be addressed by the reconciler work documented in `docs/research/reconciler/`):
+**Known gaps** (documented in architecture docs, planned for reconciler implementation):
 
-- `unmount()` is a stub — no fiber tree teardown, no instance removal, no signal disposal
-- `render()` is mount-only — no re-render, no diffing, no `prepareUpdate`/`commitUpdate` calls
-- `dispose` functions are no-ops — signal subscriptions leak
-- No `key` field on `UElement` — positional matching only
+- `unmount()` is a stub — no fiber tree teardown, no instance removal, no signal disposal. See [host-config.md](host-config.md) and [lifecycle.md](lifecycle.md).
+- `render()` is mount-only — no re-render, no diffing, no `prepareUpdate`/`commitUpdate` calls. See [reconciler.md](reconciler.md).
+- `dispose` functions are no-ops — signal subscriptions leak. See [lifecycle.md](lifecycle.md).
+- No `key` field on `UElement` — positional matching only. See [ADR-004](decisions/004-key-as-first-class-field.md).
 
 ## Architecture Documents
 
@@ -49,6 +49,8 @@ UJSX is functional but incomplete. The core primitives exist and are tested:
 | [element-factory.md](element-factory.md) | h(), createRoot(), createComponent(), Fragment, JSX runtime |
 | [reactive-layer.md](reactive-layer.md) | ReactiveRoot, reactiveComponent, reactiveElement, signals, disposal gaps |
 | [host-config.md](host-config.md) | HostConfig interface, createRoot(), mount-only rendering, reconciler gap |
+| [reconciler.md](reconciler.md) | Fiber tree, reconciliation algorithm, update scheduling, TypeBox optimizations |
+| [lifecycle.md](lifecycle.md) | Mount, update, unmount/dispose lifecycle, signal cleanup, partial tree teardown |
 | [transforms.md](transforms.md) | TransformRegistry, TransformRule, TransformContext, bi-directional transforms |
 | [events.md](events.md) | EventEnvelope, PubSubLike, UjsxEventMap |
 | [pointers.md](pointers.md) | ValuePointer, selectNode, setNode, tree navigation |
@@ -61,6 +63,8 @@ UJSX is functional but incomplete. The core primitives exist and are tested:
 | [001](decisions/001-html-agnostic-core.md) | HTML-agnostic core — no DOM-specific props |
 | [002](decisions/002-typebox-module-as-registry.md) | TypeBox Module IS the type registry |
 | [003](decisions/003-preact-signals-for-reactivity.md) | Preact signals-core for reactivity |
+| [004](decisions/004-key-as-first-class-field.md) | `key` as a first-class field on UElement — not a prop |
+| [005](decisions/005-signal-driven-updates-over-tree-diffing.md) | Signal-driven updates for props, reconciliation for structure |
 
 ## Consumer Context
 
@@ -84,18 +88,16 @@ An OpenCode plugin that provides UJSX-based template operations. Would use Trans
 
 ## Reconciler Roadmap
 
-The reconciler research in `docs/research/reconciler/` documents a phased plan to close the current gaps:
+The reconciler bridges the reactive layer to the host layer, enabling signal-driven updates and key-based children reconciliation. Architecture docs define the WHAT and WHY; research docs contain the detailed implementation plans.
 
-| Phase | Description | Status |
-|-------|-------------|--------|
-| 0 | `key` field on `UElement` | Research complete |
-| 1 | Reactive → Host bridge (fiber tree, signal-driven updates) | Research complete |
-| 2 | Key-based children reconciliation (LIS algorithm) | Research complete |
-| 3 | Unmount & dispose support | Research complete |
-| 4 | TypeBox value optimization layer | Research complete |
-| 5 | Flowgraph HostConfig implementations | Research complete |
-
-Research docs are in `docs/research/reconciler/`. Architecture docs for the reconciler will be created during the architecture phase of the SDD process, informed by this research.
+| Phase | Description | Architecture | Research |
+|-------|-------------|-------------|----------|
+| 0 | `key` field on `UElement` | [ADR-004](decisions/004-key-as-first-class-field.md), [schema.md](schema.md) | [00-KEY-FIELD-DESIGN.md](../research/reconciler/00-KEY-FIELD-DESIGN.md) |
+| 1 | Reactive → Host bridge (fiber tree, signal-driven updates) | [reconciler.md](reconciler.md), [ADR-005](decisions/005-signal-driven-updates-over-tree-diffing.md) | [01-reactive-host-bridge.md](../research/reconciler/01-reactive-host-bridge.md) |
+| 2 | Key-based children reconciliation (LIS algorithm) | [reconciler.md](reconciler.md) | [02-key-based-children-reconciliation.md](../research/reconciler/02-key-based-children-reconciliation.md) |
+| 3 | Unmount & dispose support | [lifecycle.md](lifecycle.md) | [03-unmount-dispose-support.md](../research/reconciler/03-unmount-dispose-support.md) |
+| 4 | TypeBox value optimization layer | [reconciler.md](reconciler.md) (TypeBox Optimization Layer section) | [04-typebox-optimization-layer.md](../research/reconciler/04-typebox-optimization-layer.md) |
+| 5 | Flowgraph HostConfig implementations | Downstream consumer (`@alkdev/flowgraph`), not ujsx | [05-flowgraph-host-configs.md](../research/reconciler/05-flowgraph-host-configs.md) |
 
 ## Document Lifecycle