refactor(viewer): modularize HLS playback tracking with per-panel state machine

Decompose monolithic wirePlaybackTracking() into single-responsibility modules:

- PanelState: per-panel playback state (phase, segments, buffer, quality)
- EventWiring: attaches hls.js + video element event listeners
- BufferMonitor: periodic buffer health check with timer lifecycle
- PanelRenderer: per-panel overlay rendering (phase, quality, segments)
- AggregateRenderer: global status bar aggregating all panel states
- PanelFactory: DOM panel creation + HLS/direct playback routing

Each module handles one concern: state tracking, event registration,
buffer monitoring, overlay rendering, aggregate rendering, or DOM creation.

Also fix: walk() dead code bug in m3u8DeepProbeFn where the string check
was unreachable behind the object-type guard.

__tests__/run.mjs

@@ -6,6 +6,7 @@ import './viewer_units.test.mjs';
 import './data_flow.test.mjs';
 import './viewer_integration.test.mjs';
 import './config_validation.test.mjs';
+import './hls_qa.test.mjs';
 import { run } from './runner.mjs';
 
 const result = await run();