fix(portals): match the sub-HD formats, and drop the caption on failure

Two follow-ups to the previous round, both the same rule again.

The 800-wide band still answered from the width alone, so 800x600 and
800x450 were labelled 480p — published with `api` provenance, so read as a
measurement. Sub-HD formats are now matched against known shapes with the
same 5% tolerance the height path uses, and anything unrecognised carries no
tag at all.

And "Playing from ..." survived a playback failure: the inline host stays
mounted while the diagnostic is on screen, so the page named a source for a
stream it had just reported it could not play. The caption now clears on
failure and returns when the engine produces time again.

Splits the route playback spec along the "what it does" / "what it claims"
seam and lifts the repeated active-source stub into one helper.

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

docs/architecture/vod-multi-source.md

@@ -61,10 +61,11 @@ Three rules follow, and each is enforced in code rather than by convention:
    letterboxed masters are cropped vertically — a 2.39:1 1080p film is 1920×800,
    and 800 alone is indistinguishable from a 1280×800 encode. With no width, a
    height is trusted only within 5% of a standard frame height; otherwise no tag
-   is emitted. Below the HD widths the numbers stop separating cleanly — 720
-   wide is NTSC 480p or PAL 576p depending on the height, 640 is 360p — so an
-   unrecognised shape returns nothing rather than a bucket that would be
-   published as an `api` fact.
+   is emitted. Below the HD widths the ranges stop working: 800×600
+   and 800×450 are neither 480p nor each other, and 720 wide is NTSC 480p or
+   PAL 576p depending only on the height. Those formats are therefore matched
+   rather than bucketed, and an unrecognised shape returns nothing rather than
+   a label that would be published as an `api` fact.
 
 Provenance is per-field and changes over time: at discovery a row has only
 `parsed` tags, because the `content` table stores no container, codec or audio.
@@ -368,9 +369,11 @@ the resume point at wherever playback began, so a switch an hour later rewinds
 the whole session.
 
 The caption itself appears only while a player is actually running — inline or
-a matched external session. Discovery marks a source active as the page opens,
-so gating on that alone would have the page claim "Playing from …" before Play
-was pressed, and again after the player was closed.
+a matched external session, and not while a playback diagnostic is up.
+Discovery marks a source active as the page opens, so gating on that alone
+would have the page claim "Playing from …" before Play was pressed, after the
+player was closed, and over the error screen for a stream that would not
+play.
 
 Whichever source ends up playing, the "playing" badge follows it: starting the
 route's own stream (Play, Resume, Restart, or the fallback after a pin does not