fix(encode): tag HEVC as hvc1 so AVFoundation players open the file

[LeadcodeDev]

--codec h265 wrote an mp4 that QuickTime, Safari and avconvert refuse. ffmpeg's mp4/mov muxer writes HEVC under the hev1 sample entry unless told otherwise, and Apple's stack reads only hvc1. The issue measured it: a -c copy remux that changes the tag and nothing else makes the identical bitstream play.

ffmpeg_args now derives the tag from the codec and the output extension, so it lands on the libx265 branch and the hardware one alike — hevc_videotoolbox, hevc_nvenc, hevc_qsv, hevc_amf all go through the same muxer with the same gap. mkv has no sample entry table, so it gets nothing, and no other codec is touched: prores_ks sets its own tag, and avc1 was never refused.

rustmotion concat turned out to need no change. -c copy carries the tag across, checked on two --frames segments rendered from this branch and joined: hvc1, hvc1, hvc1.

Verification — three args-level tests (both codec spellings, both branches, .mp4/.mov/uppercase/the .partial. path the encoder actually passes; absent for mkv and for every other codec), plus one that renders an HEVC mp4 and reads codec_tag_string back with ffprobe, skipping itself where ffmpeg, ffprobe or libx265 is missing. The args test was red before the fix and the two negative ones green, which is what keeps the fix from over-reaching. cargo fmt --all --check, cargo clippy --workspace --all-targets --features rustmotion/studio -- -D warnings, cargo test --workspace --features rustmotion/studio: 0 failed.

Closes #457