fix(stalker): keep live playback when switching the ITV/radio category (#1517)

Switching the Live TV or radio category in the shell context panel tore
down the Stalker player: `onStalkerCategoryClicked` cleared the selected
item for every section, and the live layout gates its player on
`selectedItem`. Xtream live (#936) and M3U groups already keep the channel
playing across a category/group switch.

- Context panel: return before `clearSelectedItem()` for `itv`/`radio`;
  VOD/series clicks still drop the open detail before navigating.
- Live layout: the category-change reset effect no longer wipes the
  playing channel's short-EPG fallback or cancels a fallback load in
  flight; only a section change (itv <-> radio) does that now.
- Regression coverage in the context panel spec, the live layout spec and
  a new web E2E scenario; docs and a `.changes/` note added.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

docs/architecture/workspace-shell.md

@@ -146,6 +146,14 @@ default. The panel header exposes a sort menu next to category search with
 synthetic "all categories" entries stay pinned before sorted provider
 categories.
 
+A category click in a LIVE section (Xtream `live`, Stalker `itv` and `radio`)
+changes only the selected category: the live layouts gate their player on the
+store's selected item, so the handler must not clear it — the channel the user
+is watching keeps playing while the sidebar re-filters (Xtream: #936; Stalker:
+`onStalkerCategoryClicked` returns before `clearSelectedItem()`). VOD and
+series category clicks do drop the open detail (`setSelectedItem(null)` /
+`clearSelectedItem()`) because they navigate to a list route.
+
 ## Search And Navigation Rules
 
 Search is shell-owned and route-aware: