fix(pipeline): link the byte-stream capsfilter after h264parse, not before - #36

…efore

Starting an AirPlay cast on macOS failed immediately, before any network
traffic:

    Selected encoder "vtenc_h264" VideoToolbox H.264 (Hardware) hw=true
    ERROR AirPlay casting failed
      e=GStreamer error: Failed to link elements 'vtenc_h264-1' and 'capsfilter1'

Capture and encoder probing both worked; the pipeline would not assemble.
`vtenc_h264` advertises `stream-format: avc` on its src pad as a fixed
string, not a list, so it cannot emit byte-stream at all. The AirPlay
chain demanded byte-stream directly from the encoder:

    encoder -> capsfilter(byte-stream) -> h264parse -> appsink

leaving an empty caps intersection. h264parse is the element that converts
avc to byte-stream, so it has to come first. `x264enc` offers
`{ avc, byte-stream }`, which is why the same chain links on Linux and why
this went unnoticed — the Miracast pipeline already uses the correct shape
(`encoder -> h264parse -> mpegtsmux`), which is why Miracast works.

Reduced, the old order reproduces the failure exactly and the new one does
not:

    $ gst-launch-1.0 videotestsrc ! vtenc_h264 \
        ! 'video/x-h264,profile=high,stream-format=byte-stream' ! h264parse ! fakesink
    WARNING: erroneous pipeline: could not link vtenc_h264-0 to h264parse0,
      vtenc_h264-0 can't handle caps video/x-h264, profile=high, stream-format=byte-stream

    $ gst-launch-1.0 videotestsrc ! vtenc_h264 ! h264parse \
        ! 'video/x-h264,stream-format=byte-stream,alignment=au' ! fakesink
    (clean)

Also drops the `profile=high` constraint. `vtenc_h264` has no profile
property and does not advertise the field, so pinning it would just move
the negotiation failure one element downstream.

`sender_pipeline.rs` (the WebRTC path) carried the same over-constraint and
is fixed alongside, though it has no callers so this is latent rather than
observed. It needs no h264parse: `rtph264pay` accepts avc and byte-stream
alike, so constraining alignment only lets the two negotiate.