formats: move media assembly out of the viewer, where it could not be reused

The trickiest reading on the disc lived in the Bevy viewer: resolving a
cutscene's voice to a continuous byte REGION of the sound stream, because the
movie voices are one XMA stream chunked into VOICE_*.slb entries whose
boundaries do not match the cues -- a cue routinely spans two chunks, so a .slb
need not hold the track its name claims.

That put the logic most likely to be re-derived incorrectly in the crate least
likely to be reused. The Godot port's exporter needs the same answers, and there
must be one implementation of them.

New `sylpheed_formats::media` owns every case where the bytes of one playable
thing are not one archive entry: segment-spanning reads, multi-sub-wave banks,
and the voice-region resolution. Callers supply bytes through a `DiscSource`
trait, so the viewer keeps its ISO/directory abstraction and a headless consumer
gets `DirectorySource` for free.

The seam is deliberate: this module returns XMA RIFFs, not PCM. Decoding means
shelling out to FFmpeg, which is native-only and a policy decision for the
consumer -- everything up to "here are the bytes that belong together" is disc
knowledge, everything after it is a codec choice.

The four moved functions were previously untested; `tests/media_disc.rs` now
pins them, including the negative the corpus paid for -- an unbound movie must
stay unvoiced rather than borrow a neighbour's clip, which was tried and played
the WRONG recording.

The algorithm is unchanged, moved verbatim (same window sizes, same fallbacks).
The new disc tests pass; the broader audio suite was not re-run in this pass.

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

tools/re-capture/screen_build_capture.sh

@@ -38,7 +38,7 @@ wide(){ [ "$(identify -format '%w' "$1" 2>/dev/null || echo 0)" -gt 1000 ]; }
     --create_profile_if_none="${SYLPH_TAG:-SylphRE}" \
     --logged_profile_slot_0_xuid="${SYLPH_XUID:-B13EBABEBABEBABE}" \
     >"$OUT/canary.stdout" 2>"$OUT/canary.stderr" & )
-sleep 8
+sleep "${LAUNCH_WAIT:-8}"
 until xdotool search --name "Xenia-canary" >/dev/null 2>&1; do
   [ -n "$(alive)" ] || { echo "EMULATOR GONE before the window appeared"; exit 4; }
   sleep 1