Address the code smells flagged in the .beam format spec:
- Write the project's actual sample rate on save instead of a hardcoded 48000
(add AudioProject::sample_rate()).
- Remove the vestigial RasterKeyframe.media_path field (it was only used by the
legacy ZIP loader, which now derives "media/raster/<id>.png" from the keyframe
id) and the dead buffer_path_at_time accessor. Backward-compatible: older files
carrying media_path deserialize fine (the field is ignored).
- Drop the unused SaveSettings fields auto_embed_threshold_bytes / force_embed_all
/ force_link_all; only large_media_mode was ever consulted. Un-prefix the now-used
`settings` parameter.
Update BEAM_FILE_FORMAT.md to match. The remaining notes (reserved MediaKind::Video,
exact-match version check) are design choices, left as-is.
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