feat: read chapters from the video description

[solrevdev]

Closes #16. This PR ships as 1.2.0.

Changes

• New chapters field with [{start, end, title}] in seconds. end is the next chapter's start, or the video's duration for the last chapter, or null for a live stream. It is [] when a video has none. It is filled with --metadata-only too, and appended as the last field so the existing order is unchanged.

• Parsing. ParseChapters reads the description, which ytx already fetches, and applies YouTube's rules:

  • the list starts at 0:00
  • it has at least three entries
  • times strictly increase
  • every chapter lasts at least 10 seconds, including the last one
  • the lines form one block

  A list that breaks a rule gives []. The block rule allows blank lines, but any other line ends the list. This stops later prose like "Follow me at 99:00" from being added as a chapter.

• Line layouts. 0:00 Intro, 0:00 - Intro, (00:00) - Intro, [1:02:03] Intro, • 3:12 | Intro, 03:12: Intro, 75:00 Intro, Intro - 12:34 and Intro (1:00:00). Both regexes are source-generated, so the code stays AOT-safe.

• Markdown sections. transcript and -f md get a ## Title heading at each chapter. A chapter with no captions of its own still gets its heading, so the outline matches chapters.

Verification

• 145 tests pass on net8.0 and net10.0. They cover 10 layouts, 6 rejections, 7 rule violations, a null end without a duration, timestamps before the list, heading placement, and golden JSON with chapters (indented) and [] (compact).
• Live on NYFGCESmikA (5h15m): 23 chapters, from Episode highlight at 0 s to the last at 18338–18951 s, matching the description. The Markdown starts each chapter with ## Title.
• Live on iG9CE55wbtY: [], with no headings.
• AOT (osx-arm64): no warnings. Output is byte-identical to JIT for -c NYFGCESmikA, -f md NYFGCESmikA and --metadata-only iG9CE55wbtY.

Left out

• Auto-generated "key moments", which need the watch page to be scraped.
• A chapter index on each segment.
• WebVTT chapter tracks.

Docs

• README: the field, the parsing rules and the example.
• CHANGELOG: a 1.2.0 entry.
• AGENTS.md: the data flow, and the rule that new fields go at the end.
• Write-up: a new section 6a in docs/investigations/2026-10-ytx-review-to-1.1.0.md.

Release

Merge with [skip ci], then run "Publish NuGet (ytx)" manually with minor to publish 1.2.0.

🤖 Generated with Claude Code

https://claude.ai/code/session_01FPou7f8nRjX78T2qR7hTjb