fix(473): gate the cancellation filter on the caller's token

Third review pass: MERGEABLE WITH NITS. Taking the one finding it asked
for before merge, plus a doc nit.

The cancellation filter added last commit had no token check, and it spans
the whole Transcode body -- including every mediator send (ffprobe via
CliWrap, media-server API calls, subtitle extraction, song-video
generation). TaskCanceledException is also what HttpClient throws on its
OWN timeout, so a real timeout in any of those was being downgraded from
an ERROR with a stack trace to a routine "Terminating HLS session"
Information line. Behaviour was unchanged (both arms return false) but the
fault signal was lost, and this repo has been bitten before by
"empty log != the event didn't happen". Now filters on
cancellationToken.IsCancellationRequested, so only genuine caller
cancellation is treated as a graceful teardown.

Doc nit: the <exception> block said cancellation "is thrown"; it is only
thrown when the token trips while the probe is in flight -- cancelling
after it completes returns normally. Now says "may propagate".

Declined the reviewer's optional suggestion to drain until a 0-return
instead of reading exactly one byte: reading exactly one byte is what
makes the guard safe BY CONSTRUCTION, since a server or proxy that
answers 206 with a wider range than requested still cannot be drained
unboundedly. 206-only was confirmed correct rather than extended to short
200s, since deciding "short" from Content-Length would reopen the
unbounded path for a chunked or Content-Length-less response.

Also records the operator's standing rule in the handoff lore: a lone
`decisions.md append-only` red is a known infra flake -- do not
investigate, rebase, amend or push to clear it; the operator reruns that
job from the UI. I violated this earlier in this PR with a
tidy-but-wrong "my entry is no longer at EOF" theory, and the rebase did
not fix it -- the job went red again on a verified pure-append diff, which
is the proof the red was never about the diff.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV.Application/Streaming/HlsSessionWorker.cs

@@ -673,12 +673,16 @@ public class HlsSessionWorker : IHlsSessionWorker
                 }
             }
         }
-        catch (Exception ex) when (ex is TaskCanceledException or OperationCanceledException)
+        catch (Exception ex) when (ex is TaskCanceledException or OperationCanceledException
+                                   && cancellationToken.IsCancellationRequested)
         {
             // a cancellation anywhere in this method (including inside the mediator sends, which sit
             // outside the inner ffmpeg try below) is a shutdown or a client disconnect, not a fault.
             // Without this it reaches the catch-all and logs a channel-level ERROR with a stack
-            // trace on every graceful teardown. (ersatztv#473 review)
+            // trace on every graceful teardown. The token check is load-bearing: TaskCanceledException
+            // is also what HttpClient throws on ITS OWN timeout, and a real timeout inside ffprobe, a
+            // media-server call or subtitle extraction must keep its ERROR-level signal rather than
+            // being downgraded to a routine teardown. (ersatztv#473 review)
             _logger.LogInformation("Terminating HLS session for channel {Channel}", _channelNumber);
 
             return false;

ErsatzTV.Core/Interfaces/Streaming/IRemoteStreamProber.cs

@@ -18,10 +18,11 @@ public interface IRemoteStreamProber
     ///     <c>true</c>, so a probe that cannot answer never prevents a tune that would have worked.
     /// </returns>
     /// <exception cref="OperationCanceledException">
-    ///     Thrown when <paramref name="cancellationToken" /> is cancelled. Caller cancellation is a
+    ///     May propagate when <paramref name="cancellationToken" /> is cancelled while the probe is
-    ///     genuine signal (shutdown / client disconnect), not a probe failure, so it propagates
+    ///     in flight. Caller cancellation is a genuine signal (shutdown / client disconnect), not a
-    ///     rather than being absorbed by the fail-open behaviour above. The prober's own internal
+    ///     probe failure, so it is not absorbed by the fail-open behaviour above. Cancelling after
-    ///     timeout does <em>not</em> throw — it fails open.
+    ///     the probe has already completed returns normally. The prober's own internal timeout does
+    ///     <em>not</em> throw — it fails open.
     /// </exception>
     Task<bool> IsAvailable(string url, CancellationToken cancellationToken);
 }

docs/handoffs/chicorytv-issue-queue.md

@@ -249,6 +249,18 @@ HARD CONSTRAINTS:
   must supersede a live run, SAY SO explicitly instead of leaving it burning. (`dispatch_workflow` is
   a different route and still works for re-triggering a **main** run.) Corrects the older "superseded
   runs drain on their own" note below: they do finish, but they hold a slot while doing it.
+- **A lone `decisions.md append-only` red is a KNOWN INFRA FLAKE — do NOTHING** (operator-stated,
+  2026-07-19). When it is the **only** red: do not investigate it, and do **not** create a new run or
+  commit to clear it — no rebase, no `--amend`, no no-op push. **The operator reruns that single job
+  from the Gitea UI.** Report it as a known flake and carry on; only if *other* jobs are red too does
+  the run deserve diagnosis. Same family as the killed-job rule below (a spurious single-job red is
+  cleared by the operator's selective rerun, never by pushing — and pushes can't be cancelled, see the
+  batching constraint above). **The trap is that a convincing local explanation is always available.**
+  On #473/PR #479 the job went red just after `main` landed its own `decisions.md` entry, so "mine is
+  no longer at EOF, I must rebase" looked airtight — I rebased, and it went red **again** on a head
+  whose diff was a verified pure EOF append with zero deleted lines. A rebase that provably satisfied
+  the gate's stated rule did not turn it green, which is the proof the red was never about the diff.
+  **Check this rule before theorising about the cause.**
 - **Do NOT gate or throttle your push on host health — trust the Gitea build queue.** The runners were
   retuned for stability (operator, 2026-07-17); queueing is the queue's job, not yours. Don't SSH to
   bumblebee to sample load/RAM before pushing, and don't hand-schedule around other sessions' runs. Batch