refactor(playback): extract diagnostic utilities

docs/architecture/m3u-playlist-module.md

@@ -994,8 +994,9 @@ player in settings.
 - `ShakaVideoSession` (`libs/ui/playback/src/lib/shaka-engine/`) owns the
   engine: lazy `import('shaka-player')` on first use (the module is a separate
   lazy chunk, ~217 KB transfer), `drm.clearKeys` configuration, an operation
-  queue + generation guard against channel-switch races, and a Shaka `5.2.2`
-  public-error boundary. The boundary version-locks its allowlisted
+  queue + generation guard against channel-switch races. The DOM-free Shaka
+  `5.2.2` public-error boundary lives in `libs/playback/util`; it version-locks
+  its allowlisted
   severity/category/code values, emits only structured sanitized
   `PlaybackDiagnosticSource.Shaka` evidence, ignores recoverable error events,
   and treats a rejected load as terminal even if its final retry error retains

docs/architecture/nx-workspace-boundaries.md

@@ -73,6 +73,11 @@ in `libs/portal/shared/util`, while reusable collection views stay in
 `libs/portal/shared/ui`. Existing injectable or stateful services in a `util`
 path are legacy debt, not precedent for new placement.
 
+Playback follows the same split: browser and Angular player integration stays
+in `libs/ui/playback`, while DOM-free diagnostic contracts and classifiers live
+in `libs/playback/util` and receive browser capability checks as explicit
+probes.
+
 ## Project Tags
 
 Every Nx project keeps one tag from each family in `project.json`: