docs(21-05): create summary and update state for code splitting & performance

Add 21-05-SUMMARY.md documenting route-based code splitting results
(582 KB -> 454 KB main chunk + route-level chunks) and PlanTasksFetcher
useEffect fix. Update STATE.md to plan 5/6 with new decisions.

.planning/STATE.md

@@ -10,18 +10,18 @@ See: .planning/PROJECT.md (updated 2026-02-04)
 ## Current Position
 
 Phase: 21 of 21 (Polish & Integration) - In progress
-Plan: 4 of 6 in current phase
-Status: Completed 21-04 (Subscription Error Handling)
-Last activity: 2026-02-05 - Completed 21-04-PLAN.md (subscription error toasts)
+Plan: 5 of 6 in current phase
+Status: Completed 21-05 (Code Splitting & Data-Fetching Performance)
+Last activity: 2026-02-05 - Completed 21-05-PLAN.md (route code splitting + PlanTasksFetcher fix)
 
 Progress: █████████░ 99%
 
 ## Performance Metrics
 
 **Velocity:**
-- Total plans completed: 73
+- Total plans completed: 74
 - Average duration: 3 min
-- Total execution time: 206 min
+- Total execution time: 209 min
 
 **By Phase (v1.0):**
 
@@ -247,6 +247,8 @@ Recent decisions affecting current work:
 - 21-04: duration: Infinity keeps error toast visible until user dismisses — persistent connection feedback
 - 21-04: No reconnection logic — tRPC SSE httpSubscriptionLink handles reconnection via EventSource retry
 - 21-04: SpawnArchitectDropdown already covered by 21-01 — verification task was no-op
+- 21-05: autoCodeSplitting: true on TanStackRouterVite plugin — automatic route splitting without manual .lazy.tsx files
+- 21-05: PlanTasksFetcher onTasks moved to useEffect to prevent setState-during-render loops
 
 ### Pending Todos
 
@@ -269,5 +271,5 @@ None.
 ## Session Continuity
 
 Last session: 2026-02-05
-Stopped at: Completed 21-04 (Subscription Error Handling)
+Stopped at: Completed 21-05 (Code Splitting & Data-Fetching Performance)
 Resume file: None