fix(portals): treat an undetermined audio language as unknown (#1689)

* fix(portals): treat an undetermined audio language as unknown

ICU 78, which Electron 43 and Node 26 ship, canonicalizes the ffprobe
marker `und` to the subtag `und` instead of an empty one. The source
metadata then recorded it as a stated language, so a switch between a
copy tagged `und` and one tagged English raised the "dub may differ"
warning. CI's older ICU hid this: the existing spec only fails on the
newer runtime.

Decline `und` explicitly, and cover `und-US` plus the dub comparison.

* test(portals): cover undetermined language across runtimes

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Co-authored-by: 4gray <serega05@gmail.com>

docs/architecture/vod-multi-source.md

@@ -776,6 +776,11 @@ tag a language at all, so in practice it is usually silent — which is the
 honest state, and the same one the rest of this feature takes when it does not
 know.
 
+Undetermined language tags (`und`, including regional forms such as `und-US`)
+are explicitly treated as unknown after normalization. This must hold whether
+the runtime's `Intl.Locale` returns no language or preserves `und`: neither
+case establishes a factual dub difference against a known language.
+
 Switching sources through `startResolvedPlayback` closes the external session
 it LAUNCHED first — tracked separately so refreshes and overlapping handoffs
 cannot disown it within the same route session. The retained identity is ignored