fix(explorer): resolve #830 — Maximum update depth exceeded on Temporal panel open

Two independent render loops were causing the Temporal panel to remain
stuck on 'Loading temporal...' in npm run dev:

Loop 1 — diagnostics state churn (GraphWorkspace.tsx):
  handleDiagnosticsChange unconditionally called setGraphDiagnosticsState
  with a new object on every invocation. buildEffectAvailability (called
  inside GraphCanvas's diagnostics useEffect) always returns a new object,
  so setGraphDiagnosticsState was called on every effect run, creating a
  cycle: setGraphDiagnosticsState  graphDiagnosticsState new
  diagnosticsSnapshot new  pluginContext new  handleInteractionStateChange
  new  GraphCanvas re-renders  diagnostics effect fires again.

  Fix: before calling setGraphDiagnosticsState, compare the incoming
  diagnostics field-by-field against the last accepted snapshot via a ref
  (lastDiagnosticsRef). All effectAvailability entries, edgeClasses.updatedAt,
  structureLayer.cacheKey/lastDrawAt/enabled, and distanceVisual identity
  must differ for a state update to proceed. The ref approach avoids
  scheduling a re-render at all, rather than bailing out inside a functional
  updater after the render has already been committed.

Loop 2 — scrubberTime churn (GraphWorkspace.tsx + GraphWorkspaceShell.tsx):
  TimelinePanel.tsx calls onTimeChange(defaultTime) whenever its useEffect
  re-runs. React 18 concurrent mode re-runs effects with structurally-new
  Date objects for the same timestamp when speculative renders discard
  useMemo caches, causing setScrubberTime to be called repeatedly with a
  new Date that has the same millisecond value — triggering temporalState
  churn, the diagnostics effect, and eventually the same loop.

  Fix: wrap setScrubberTime in an onTimeChange useCallback that compares the
  incoming time's millisecond value against the last sent value (via
  lastScrubberMsRef). Redundant calls with the same timestamp are dropped
  before reaching setScrubberTime. Stable useCallback identity also prevents
  TimelinePanel's useEffect from re-firing solely due to prop identity churn.

Both fixes applied to GraphWorkspace.tsx and identically to
GraphWorkspaceShell.tsx which has the same pattern.

Verified:
- npm run dev: 0 'Maximum update depth exceeded' errors
- Temporal panel renders with real data in dev mode
- Effects and Neighbors panels unaffected
- npm run build + preview: identical behavior, 0 errors
- All 42 frontend tests pass (34 graph-workspace, 1 graph-store, 7 plugin-registry)

explorer/src/workspaces/GraphWorkspace/GraphWorkspaceShell.tsx

@@ -325,6 +325,17 @@ export function GraphWorkspaceShell() {
   const [activeNodeCount, setActiveNodeCount] = useState<number | null>(null);
   const [temporalBounds, setTemporalBounds] = useState<TemporalBounds | null>(null);
   const [scrubberTime, setScrubberTime] = useState<Date | null>(null);
+  // Deduplicates setScrubberTime calls by millisecond value — same fix as
+  // GraphWorkspace.tsx (issue #830).
+  const lastScrubberMsRef = useRef<number | null>(null);
+  const onTimeChange = useCallback((time: Date) => {
+    const ms = time.getTime();
+    if (ms === lastScrubberMsRef.current) {
+      return;
+    }
+    lastScrubberMsRef.current = ms;
+    setScrubberTime(time);
+  }, []);
   const [loadingProgress, setLoadingProgress] = useState<GraphLoadProgress | null>(null);
   const [isGraphStageReady, setIsGraphStageReady] = useState(false);
   const [layoutStatus, setLayoutStatus] = useState<GraphLayoutStatus>({
@@ -632,7 +643,7 @@ export function GraphWorkspaceShell() {
 
       <Suspense fallback={<TimelineFallback min={temporalBounds?.min ?? null} max={temporalBounds?.max ?? null} />}>
         <TimelinePanel
-          onTimeChange={setScrubberTime}
+          onTimeChange={onTimeChange}
           minDate={temporalBounds?.min ?? undefined}
           maxDate={temporalBounds?.max ?? undefined}
         />