CapRust 0.3.0-alpha.1 — audio, seek, hardware encoders

Third public alpha. Audio pipeline rewritten, seek made usable, and
hardware encoders wired in.

Added

• Hardware-accelerated export (NVENC, AMF). The export dialog
  probes the running ffmpeg build on first open and offers only the
  encoders that actually open on this machine. CPU H.264/H.265/AV1
  remain always available. HEVC and AV1 output carry the correct
  mp4 tags (hvc1, av01). Measured 5-15x faster on an RTX 2060.
• Pre-rendered audio PCM cache. A background ffmpeg process
  renders the whole project's audio mix to a stable PCM file in
  %TEMP%. The play path prefers this cache over the preview's own
  PCM output, so audio survives a preview respawn instead of
  restarting on every edit and seek.
• Seek optimization in the preview renderer. plan_from_project
  now takes the current playhead and rewrites each input to
  -ss/-t so ffmpeg demuxer-seeks instead of decoding from t=0.
  A seek into the second half of a 3-minute project goes from 5-10 s
  to ~200-500 ms. Projects with transitions fall back to the old
  output-side seek.
• Missing-media relink flow. On project load, if any media file
  is gone, a dialog lists the missing items, offers a folder picker,
  and matches files by basename (case-insensitive, depth-capped
  scan). The whole batch runs as one undoable RelinkManyCommand.
• Media bin shows a red "Missing" badge on cards whose source file
  is gone.
• Timeline clips without a source file are drawn with a red
  diagonal hatch so a broken clip cannot hide under its normal
  fill colour.
• Paused seek now spawns a one-shot preview render at the new
  playhead. Previously a timeline click while paused moved the
  playhead but left the old frame on screen and no renderer alive.

Fixed

• Final audio mix was attenuating clips cumulatively. A chain
  of N sequential amix filters with the default normalize=1
  divided the signal by 2 at each stage, so the first clip on a
  17-clip project played at roughly 1/2^17 of the last. Replaced
  with a single amix across all inputs at normalize=0. The
  opening clip is now at full level and a lone m4a no longer plays
  ~10x louder than video-embedded audio.
• Video frame rate and playback stayed silent on a seek past the
  first clip. PTS was not reset before the filtergraph's trim, so
  the trim saw an empty window and selected zero frames. Reset
  before trim in both video and audio chains.
• RenderPlan::total_duration_sec was zero when a seek was applied
  because it had already been derived from the seek-shifted clips;
  subtracting again collapsed the base black / silence bed to 0 s.
• The 650 ms A/V delay compensation is now skipped on
  seek-optimized plans. The fixed value pushed video behind audio
  whenever the audio was cache-fed and the video was already
  seeked.

Changed

• Cargo.lock regenerated; workspace version bumped to
  0.3.0-alpha.1.
• ExportSettings.codec_index values 3..=8 now select NVENC and
  AMF variants. 0/1/2 keep their old meaning. Existing projects
  load unchanged.

Known issues

• No installer yet; download the nightly ZIP and extract it.
• FFmpeg is not bundled. First run offers to download a BtbN build
  (~110 MB) or to point at an existing install. NVENC and AMF
  require a build that was compiled with --enable-nvenc /
  --enable-amf; the BtbN and gyan.dev builds both satisfy that.
• Windows only.
• Text overlay rotation is stored in the model but neither rendered
  nor exposed in the UI.
• MyMemory translation quality is below DeepL.
• Preview A/V sync has a residual 100-300 ms lead/lag on some
  hardware. Tunable via CAPRUST_AV_DELAY_MS; a UI slider is
  planned.