feat(video): honor URL t=/start= timestamp as a playback start offset

[kfox]

What

Playing a URL with a timestamp previously ignored it and started at 0:00. Now a
quick-playback URL begins at its own timestamp, with no new CLI flag — the
lowest-friction path stays low-friction:

c64cast 'https://youtu.be/<id>?t=1m30s'

?t= / &start= / #t= are honored, accepting 90, 90s, 1m30s, 1h2m3s.

How

• quickcast._parse_start_offset parses the timestamp off the original URL
  (the resolved stream URL doesn't carry it) and sets the scene's start offset.
• New SceneCfg.start_s (video-only; negative rejected, set-on-non-video
  rejected) threads through VideoScene into AVFileSource.
• AVFileSource seeks the container to the keyframe at/just-before start_s
  (whole-container seek in AV_TIME_BASE microseconds, backward) before the
  demux thread starts; the audio peak-scan container seeks too so normalization
  reflects the played portion.
• The demux loop rebases every video frame's PTS by the first decoded frame so
  playback restarts at ~0 and tracks the from-0 audio/wall clock. The no-seek
  path is unchanged (offset ≈ 0).

Applied as a PyAV seek, not a yt-dlp download-section: no temp download, and
it also covers direct media URLs and config-driven [[scenes]] videos
(start_s = N). Accuracy is keyframe-granular (start lands within one GOP);
A/V stay aligned. Exact-to-the-second start (decode-and-discard) is left as a
future refinement.

Docs + schema

CLAUDE.md, docs/usage.md, and the example TOML document start_s; the JSON
schema is regenerated.

Tests

• Timestamp parsers (_parse_timestr / _parse_start_offset): query params,
  fragment, h/m/s forms, garbage → None.
• classify_url sets start_s from a URL timestamp.
• start_s validation: accepted on video, negative rejected, rejected on
  non-video.
• A fake-container demux unit test asserting the PTS rebase (no PyAV).

Full suite green; schema in sync.

[codecov]

Codecov Report

❌ Patch coverage is 85.41667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.54%. Comparing base (9855456) to head (77771cb).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
c64cast/video.py	30.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
+ Coverage   79.43%   79.54%   +0.10%     
==========================================
  Files          68       68              
  Lines       12770    12817      +47     
  Branches     1877     1889      +12     
==========================================
+ Hits        10144    10195      +51     
+ Misses       2193     2185       -8     
- Partials      433      437       +4     

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