fix(mp4): refuse a video track with no resolved dimensions

Resolution::pixels() returned (0, 0) for Unknown, and the MP4 sink wrote
it verbatim into tkhd (ISO/IEC 14496-12 8.3.2) and VisualSampleEntry
(12.1.3). Both fields are MANDATORY there, so unlike Matroska — which
omits the optional PixelWidth/PixelHeight elements — MP4 has nothing to
leave out. The result was a structurally complete file that passes every
container check, declares a 0x0 video track, cannot be rendered, and is
written with no error anywhere.

WHY IT WAS POSSIBLE, which is the part worth keeping:

pixels() previously fabricated 1920x1080 for Unknown. That was wrong but
playable, so this sink never needed a guard and the absence of one was
invisible. Changing the sentinel to (0, 0) moved the defect instead of
removing it — a zero PAIR still reads as a usable value, so the sink
stored it and serialised it.

The accessor's doc comment then ENUMERATED the callers it believed were
safe: "the Matroska sink omits the optional elements, the VobSub writer
omits its size: line, and no caller divides by either dimension." Two of
those three are true. MP4 was not on the list because MP4 has no guard
at all, and a prose list cannot enforce itself. mkv.rs's own comment
even states the principle — "the check belongs in the one accessor
rather than in each caller that remembered to write it" — and
labels/mod.rs still carried its own duplicate Unknown test long after
the accessor took that job over.

So: pixels() now returns Option. Not because Option is tidier, but
because every caller genuinely needs a DIFFERENT answer and the compiler
is the only thing that reliably makes them choose one. Matroska and the
metadata sinks take unwrap_or((0, 0)) with the reason stated at each
site; the VobSub path degrades to a palette-only .idx; MP4 fails with
E_MP4_UNKNOWN_RESOLUTION (9055).

Six call sites, not the five my first grep showed — I piped it through
`head` and acted on a truncated list. The compiler caught the sixth.
That is the same mistake as trusting a lens that reported silence.

src/mux/mkv.rs

@@ -385,7 +385,9 @@ impl MkvTrack {
         // that used to sit here existed only because `pixels()` fabricated a
         // 1920x1080 default; it does not any more, so the check belongs in the
         // one accessor rather than in each caller that remembered to write it.
-        let (w, h) = v.resolution.pixels();
+        // None -> 0, and the writer already omits the optional
+        // PixelWidth/PixelHeight elements on 0 (RFC 9559 5.1.4.1.28-29).
+        let (w, h) = v.resolution.pixels().unwrap_or((0, 0));
         let (num, den) = v.frame_rate.as_fraction();
         let default_duration_ns = if num > 0 {
             (1_000_000_000u64 * den as u64) / num as u64