Implement CE-WP-0003 T01-T08: form binding + visual guide overlay

T01 EvidenceLink/EvidenceSet types
  - src/shared/evidence-link.ts: status (§2.4), relation (§2.5), target
  - src/shared/evidence-set.ts: ordered group + activeEvidenceItemId
  - enum-conformance test parses SharedContracts.md and asserts the
    runtime lists match exactly

T02 Binding service + in-memory link repo + active-state machine
  - src/binder/repos/in-memory-links.ts: Map-backed EvidenceLinkRepository
  - src/binder/services/bindings.ts: link/unlink/list/update/setActive
    emitting §4 EvidenceLinkCreated / EvidenceLinkUpdated /
    EvidenceItemActivated
  - src/binder/state/active.ts: (target, evidence, annotation) reducer
    + ActiveStateProvider + useActiveState hook
  - extended engine/events/types.ts with EvidenceLinkCreated,
    EvidenceLinkUpdated, FormFieldActivated payloads

T03 Rect registry (SharedContracts §7 — contract FROZEN)
  - src/binder/visual-guide/rect-registry.ts: register/getRect/subscribe
    + invalidate + getVersion for useSyncExternalStore
  - events.ts: scroll/resize/focus pumps via window + ResizeObserver +
    IntersectionObserver, rAF-throttled
  - react-hooks.ts: RectRegistryProvider, useRegisterRect(kind,id,ref),
    useRectRegistryVersion

T04 Form schema + renderer
  - src/app/forms/demo-schema.ts: text/textarea/date minimal schema
  - src/binder/FormRenderer.tsx: renders schema, each field registers
    as rect kind="field"; active field gets aria-current="true"
  - placed in binder/ (not work/) because work cannot import binder per
    DependencyMap.md §2 and the renderer needs the rect-registry hook;
    workplan T04 was amended in-place to document this

T05 Side-by-side Forms layout + click-to-link
  - src/app/forms/FormsApp.tsx + src/app/App.tsx top-bar router with
    hash route #/forms/demo
  - BinderProvider mounted at app root so links survive tab switching
  - stage-evidence-then-click-field linking interaction with banner
    + per-field link-count chip

T06 Active-evidence cycling
  - src/app/forms/ActiveEvidenceChips.tsx: chips per active target,
    Tab cycles natively, first chip auto-activates on field focus,
    each chip registers as rect kind="evidence-card"
  - ScrollBridge in FormsApp wires activeAnnotationId to viewer scroll
  - EvidenceSidebar + EvidenceStrip highlight the active item via the
    new useLastActivatedEvidence hook in work/EngineContext

T07 SVG visual-guide overlay
  - src/binder/visual-guide/Overlay.tsx: single fixed-positioned SVG,
    draws field→card and card→highlight bezier curves for the active
    triple, rAF-throttled via the registry
  - src/anchor exposes getHighlightClientRects(annotationId); the
    spike viewer wraps highlights in [data-highlight-id] so the helper
    can locate them
  - src/app/forms/HighlightRectBridge.tsx: registers the active
    annotation's rect via that helper

T08 End-to-end test (PRD scenario steps 5-9)
  - tests/integration/forms-overlay-e2e.dom.test.tsx: full path from
    Review-mode capture through Forms-mode link to active triple +
    aria-current assertions + 2 SVG paths in the overlay
  - additional integration coverage: forms-link-flow + forms-active-cycling

Gates: typecheck ✓ · lint ✓ · build ✓ · 152/152 tests across 21 files.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/binder/state/active.ts

@@ -0,0 +1,171 @@
+/**
+ * Active state machine + React context for the form-binding flow.
+ *
+ * Tracks the `(activeTarget, activeEvidenceItemId, activeAnnotationId)`
+ * triple that the SVG visual guide and the viewer adapter both depend on.
+ *
+ * Transitions:
+ *   - `focusTarget(target)` — clears the active evidence, emits
+ *     `FormFieldActivated`.
+ *   - `setActiveEvidence(evidenceItemId, annotationId?)` — sets active
+ *     evidence (and optionally the active annotation derived from it),
+ *     emits `EvidenceItemActivated` with `source="form-field"`. The
+ *     binding-service helper does the same; the state machine owns the
+ *     React-facing source of truth.
+ *   - `clear()` — drops everything back to undefined.
+ *
+ * The state itself is a small immutable record (so React equality checks
+ * stay simple). All mutations go through a single reducer.
+ */
+
+import {
+  createContext,
+  createElement,
+  useCallback,
+  useContext,
+  useEffect,
+  useMemo,
+  useReducer,
+  useRef,
+  type ReactNode,
+} from "react";
+
+import type { EvidenceTarget } from "@shared/evidence-link";
+import type { AnnotationId, EvidenceItemId } from "@shared/ids";
+
+import type { EventBus } from "@engine/events";
+
+export interface ActiveState {
+  readonly activeTarget: EvidenceTarget | null;
+  readonly activeEvidenceItemId: EvidenceItemId | null;
+  readonly activeAnnotationId: AnnotationId | null;
+}
+
+export const EMPTY_ACTIVE_STATE: ActiveState = {
+  activeTarget: null,
+  activeEvidenceItemId: null,
+  activeAnnotationId: null,
+};
+
+type Action =
+  | { type: "focus-target"; target: EvidenceTarget }
+  | {
+      type: "set-active-evidence";
+      evidenceItemId: EvidenceItemId;
+      annotationId: AnnotationId | null;
+    }
+  | { type: "clear" };
+
+function reducer(state: ActiveState, action: Action): ActiveState {
+  switch (action.type) {
+    case "focus-target":
+      // Focusing a target resets the active evidence — a different field
+      // means a different evidence set.
+      if (
+        state.activeTarget?.targetType === action.target.targetType &&
+        state.activeTarget?.targetId === action.target.targetId
+      ) {
+        return state;
+      }
+      return {
+        activeTarget: action.target,
+        activeEvidenceItemId: null,
+        activeAnnotationId: null,
+      };
+    case "set-active-evidence":
+      return {
+        activeTarget: state.activeTarget,
+        activeEvidenceItemId: action.evidenceItemId,
+        activeAnnotationId: action.annotationId,
+      };
+    case "clear":
+      return EMPTY_ACTIVE_STATE;
+  }
+}
+
+export interface ActiveStateApi {
+  readonly state: ActiveState;
+  focusTarget(target: EvidenceTarget): void;
+  setActiveEvidence(
+    evidenceItemId: EvidenceItemId,
+    annotationId?: AnnotationId | null,
+  ): void;
+  clear(): void;
+}
+
+const ActiveStateContext = createContext<ActiveStateApi | null>(null);
+
+export interface ActiveStateProviderProps {
+  readonly bus: EventBus;
+  readonly children: ReactNode;
+}
+
+/**
+ * React provider for the binder's active-state machine. Mounts inside the
+ * EngineProvider so it can wire `bus` from the engine.
+ */
+export function ActiveStateProvider(props: ActiveStateProviderProps) {
+  const [state, dispatch] = useReducer(reducer, EMPTY_ACTIVE_STATE);
+  const stateRef = useRef(state);
+  useEffect(() => {
+    stateRef.current = state;
+  }, [state]);
+
+  const focusTarget = useCallback(
+    (target: EvidenceTarget) => {
+      const previousTarget = stateRef.current.activeTarget;
+      const samePrevious =
+        previousTarget?.targetType === target.targetType &&
+        previousTarget?.targetId === target.targetId;
+      if (samePrevious) return;
+      props.bus.emit({
+        type: "FormFieldActivated",
+        target,
+        ...(previousTarget !== null ? { previousTarget } : {}),
+      });
+      dispatch({ type: "focus-target", target });
+    },
+    [props.bus],
+  );
+
+  const setActiveEvidence = useCallback(
+    (evidenceItemId: EvidenceItemId, annotationId?: AnnotationId | null) => {
+      props.bus.emit({
+        type: "EvidenceItemActivated",
+        evidenceItemId,
+        source: "form-field",
+      });
+      dispatch({
+        type: "set-active-evidence",
+        evidenceItemId,
+        annotationId: annotationId ?? null,
+      });
+    },
+    [props.bus],
+  );
+
+  const clear = useCallback(() => {
+    dispatch({ type: "clear" });
+  }, []);
+
+  const value = useMemo<ActiveStateApi>(
+    () => ({ state, focusTarget, setActiveEvidence, clear }),
+    [state, focusTarget, setActiveEvidence, clear],
+  );
+
+  return createElement(ActiveStateContext.Provider, { value }, props.children);
+}
+
+export function useActiveState(): ActiveStateApi {
+  const ctx = useContext(ActiveStateContext);
+  if (!ctx) {
+    throw new Error("useActiveState must be used inside <ActiveStateProvider />");
+  }
+  return ctx;
+}
+
+/**
+ * Pure reducer + initial state, exported so the headless tests can verify
+ * transitions without spinning up React.
+ */
+export const __test = { reducer, EMPTY_ACTIVE_STATE };