More real media formats: FLAC-in-Ogg, an ImageDecoder, and an MSE timeline

Three more page-observable media capabilities on top of last release's WebCodecs and FLAC work. First, FLAC carried in an Ogg container (.oga / .ogg) now decodes: a small Ogg page demuxer lifts the FLAC logical stream back into native FLAC bytes and hands it to the same bit-exact decoder, so decodeAudioData reads Ogg-FLAC as well as raw FLAC and WAV. Second, WebCodecs gains a working ImageDecoder: it decodes an encoded image (PNG, GIF, JPEG or WebP) to a real RGBA VideoFrame using the engine's own image decoders, so the modern decode-an-image-off-the-main-thread path produces actual pixels a page can read back and draw. Third, Media Source Extensions gains its timeline controls: timestampOffset shifts the presentation time of everything a SourceBuffer appends, and appendWindowStart / appendWindowEnd clip coded frames to a window - the two knobs adaptive players use to splice and trim streams. Every piece is verified: Ogg-FLAC decodes bit-exactly against ffmpeg, the ImageDecoder returns the exact known pixels, and the MSE timeline is checked against the real buffered ranges. Media moves 45 to 48; the overall holds at 57.