refactor: parallel coverage collector (~30-60s vs 8min) (#272)

## Summary

Redesigned frontend coverage collector with 7 parallel browser contexts.
Coverage collector runs on master pushes only (skipped on PRs).

### Architecture
7 groups run simultaneously via `Promise.allSettled()`:
- G1: Home + Customizer
- G2: Nodes + Node Detail
- G3: Packets + Packet Detail
- G4: Map
- G5: Analytics + Channels + Observers
- G6: Live + Perf + Traces + Globals
- G7: Utility functions (page.evaluate)

### Speed gains
- `safeClick` 500ms → 100ms
- `navHash` 150ms → 50ms
- Removed redundant page visits and E2E-duplicate interactions
- Wall time = slowest group (~30-60s estimated)

### 821 lines → ~450 lines
Each group writes its own coverage JSON, nyc merges automatically.

### CI behavior
- **PRs:** Coverage collector skipped (fast CI)
- **Master:** Coverage collector runs (full synthetic user validation)

Co-authored-by: you <you@example.com>

.github/workflows/deploy.yml

@@ -181,11 +181,10 @@ jobs:
         run: |
           BASE_URL=http://localhost:13581 node test-e2e-playwright.js 2>&1 | tee e2e-output.txt
 
-      # DISABLED: Coverage collector takes 8+ min. E2E tests extract window.__coverage__ directly.
-      # - name: Collect frontend coverage
-      #   if: success()
-      #   run: |
-      #     BASE_URL=http://localhost:13581 node scripts/collect-frontend-coverage.js 2>&1 | tee fe-coverage-output.txt || true
+      - name: Collect frontend coverage (parallel)
+        if: success() && github.event_name == 'push'
+        run: |
+          BASE_URL=http://localhost:13581 node scripts/collect-frontend-coverage.js 2>&1 | tee fe-coverage-output.txt || true
 
       - name: Generate frontend coverage badges
         if: success()