perf(07-01): eliminate redundant lookups in SetQueue Phase 2

- Add phase1Meta parameter to resolveRemainingTracks
- Filter out already-resolved paths before lookupTrackMetaBatch call
- Merge Phase 1 results into Phase 2 lookup map
- Pass batchMeta from SetQueue call site to resolveRemainingTracks
- For 1000-track queue with Phase 1 resolving 50, Phase 2 now queries 950 instead of 1000

.planning/phases/07-backend-performance/07-02-SUMMARY.md

@@ -0,0 +1,91 @@
+---
+phase: 07-backend-performance
+plan: 02
+subsystem: ui
+tags: [performance, startup, deferred-loading, dom-ready, wails]
+
+# Dependency graph
+requires:
+  - phase: 06-sql-consolidation-code-quality
+    provides: stable frontend store and library data access patterns
+provides:
+  - Deferred LibraryStore eagerFetch — app shell renders before backend data roundtrips
+affects: [08-frontend-polish]
+
+# Tech tracking
+tech-stack:
+  added: []
+  patterns: [deferred-initialization via DOMContentLoaded event]
+
+key-files:
+  created: []
+  modified:
+    - frontend/src/store/library-store.ts
+
+key-decisions:
+  - "DOMContentLoaded over load event — fires earlier (after HTML parsed) without waiting for all resources, still defers past module evaluation"
+
+patterns-established:
+  - "Deferred singleton initialization: singleton constructors should not fire async work; defer to DOM ready events"
+
+requirements-completed: [PERF-03]
+
+# Metrics
+duration: 1min
+completed: 2026-03-05
+---
+
+# Phase 7 Plan 2: Defer Library Data Loading Summary
+
+**Deferred LibraryStore eagerFetch from constructor to DOMContentLoaded event, ensuring app shell renders instantly before 4 backend data roundtrips begin**
+
+## Performance
+
+- **Duration:** 1 min
+- **Started:** 2026-03-05T01:53:27Z
+- **Completed:** 2026-03-05T01:54:49Z
+- **Tasks:** 1
+- **Files modified:** 1
+
+## Accomplishments
+- Removed `eagerFetch()` call from LibraryStore constructor so module evaluation no longer triggers 4 backend roundtrips
+- Added `deferEagerFetch()` method that waits for `DOMContentLoaded` event (or calls immediately if DOM already parsed)
+- App shell now renders before data fetching competes for resources
+- All 4 data types (tracks, albums, artists, genres) still eagerly loaded once DOM is ready
+- Post-scan invalidation behavior unchanged — `invalidate()` still calls `eagerFetch()` directly
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **Task 1: Defer eagerFetch from constructor to post-DOM-ready** - `cd98ad6` (perf)
+
+## Files Created/Modified
+- `frontend/src/store/library-store.ts` - Removed eagerFetch from constructor, added deferEagerFetch with DOMContentLoaded listener
+
+## Decisions Made
+- Used `DOMContentLoaded` instead of `load` event — fires earlier (after HTML parsed, before stylesheets/images finish) which minimizes delay in data availability while still deferring past the initial module evaluation. The `load` event would unnecessarily wait for all resources before beginning data fetches.
+
+## Deviations from Plan
+
+None - plan executed exactly as written.
+
+## Issues Encountered
+None
+
+## User Setup Required
+None - no external service configuration required.
+
+## Next Phase Readiness
+- Plan 02 complete — deferred library loading implemented
+- Plan 01 (lazy module loading) may still be pending
+- Frontend data loading is now deferred to post-DOM-ready, providing instant app shell render
+
+## Self-Check: PASSED
+
+- [x] `frontend/src/store/library-store.ts` exists
+- [x] Commit `cd98ad6` exists in git history
+
+---
+*Phase: 07-backend-performance*
+*Completed: 2026-03-05*