1. -profile:v main + -level:v 4.0 to avoid Chrome's HW decoder path that
failed with "VaapiWrapper: failed initializing for h264 high" on Linux.
2. setparams to rewrite HDR HEVC color metadata to SDR Rec.709 so browsers
don't reject wide-gamut output.
3. serveRange caps `want` by estimated final size (not current). ReadAt
blocks until ffmpeg catches up — that's the right behaviour. Returning
RangeEnd inmediato was making the browser abort with "Format error".
4. Debug log on every range_req.
27fe84f2a0