perf(stalker): skip wasted series-seasons request for non-series items (#1241)

`setSelectedItem` mirrored every selection's id into `selectedSerialId`, and
`serialSeasonsResource` fires a `get_ordered_list&type=series&movie_id=<id>`
portal request on every change of that id. Opening any Stalker detail page —
plain VOD, vclub items with embedded `series[]` (whose result
`mapRegularSeriesSeasons` discards), Ministra `is_series` items, and ITV
channel clicks — therefore issued a pointless request, on every entry path
(browse, favorites, recent, dashboard, search).

Set `selectedSerialId` only when `selectedContentType === 'series'`, clearing
it otherwise. The gate is deliberately on content type alone, not item shape:
under the `series` content type `serialSeasonsResource` is the only episode
source (the detail templates render `<app-stalker-series-view />` with no
`vodWithSeries` input, and `isVodSeries()` requires content type `vod`), so
gating on `is_series`/`series[]` would leave a series-section item carrying
either field with a silently empty episode list.

Adds selection-state and request-level regression coverage, and corrects a
stale invariant in the Stalker architecture docs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/architecture/stalker-portal.md

@@ -271,6 +271,14 @@ Stalker has multiple real-world data shapes. The current implementation supports
 
 - Seasons come from API resource (`serialSeasonsResource`).
 - Episodes are derived from season payload.
+- This is the only mode that sets `selectedSerialId`, which is what drives
+  `serialSeasonsResource`. It is set purely from `selectedContentType ===
+'series'` — the `series` detail branch renders `<app-stalker-series-view />`
+  with no `vodWithSeries` input, so the API resource is its only episode
+  source and the fetch must never be gated on item shape.
+- Modes 2 and 3 below are always opened under the `vod` content type, which
+  leaves the id unset — otherwise every VOD detail open would fire a
+  `get_ordered_list&type=series` request whose result is discarded.
 
 2. VOD with Embedded `series[]`:
 

docs/architecture/stalker-store-api-baseline.md

@@ -123,7 +123,15 @@ Consumer directories sampled:
 
 ## Invariants to Preserve During Refactor
 
-- Selection IDs (`selectedVodId`, `selectedSerialId`, `selectedItvId`) are synchronized in `setSelectedItem`.
+- Selection IDs (`selectedVodId`, `selectedItvId`) are synchronized in `setSelectedItem`.
+- `selectedSerialId` is set only when `selectedContentType` is `series`, and is
+  cleared for every other content type. `serialSeasonsResource` fires a
+  `get_ordered_list&type=series` portal request on each change of that id, and
+  it is the only episode source for a `series` selection — while VOD-context
+  shapes (embedded `series[]`, Ministra `is_series`) resolve their episodes
+  elsewhere, so carrying the id there only wastes a request. The gate must stay
+  on content type alone: gating it on item shape would leave a series-section
+  item that happens to carry `is_series`/`series[]` with an empty episode list.
 - `setSelectedCategory(...)` resets `page` to `0`.
 - `getPaginatedContent()` and `getCategoryResource()` always return arrays,
   even when the underlying request fails.