feat(playlist): identify playlists and list members via yt-dlp

[cmglezpdev]

What this changes

veta can now recognize a playlist?list= URL and list its members via yt-dlp --flat-playlist -J, with domain helpers for playlist directory naming (pl-<slug>-<id>) and stable NN- position prefixes. Nothing user-visible yet.

Why

Foundation slice of playlist support: identity + listing, isolated behind a new PlaylistSourcePort so the single-video pipeline stays untouched. Watch URLs with &list= remain single-video by design.

How to verify

pnpm typecheck
pnpm vitest run src/adapters/ytdlp src/domain/playlist --exclude '**/.claude/**'

The sanitized real listing sample this is based on lives at docs/examples/yt-dlp-flat-playlist.json.

Checklist

• pnpm typecheck and pnpm test pass
• Commits follow Conventional Commits

Scope

• Includes: URL identification, member listing (position = array index + 1), unavailable-entry detection (defensive, see docs), explicit maxBuffer, fixtures.
• Excludes: store records, orchestration, CLI. Unavailable-member shape unverified against a real payload (flagged in docs/03-data-sources.md).

Line count note: 630 authored lines; the rest of the diff is fixture JSON (docs/examples/, __fixtures__/).

Chain Context

Field	Value
Chain	playlist-support
Tracker PR	#40
Position	1 of 6
Base	feat/playlist-support
Depends on	None
Follow-up	PR 2 (store records)
Review budget	630 authored (+616 fixture JSON) / 400

Chain Overview

main
 └── #40 tracker: feat/playlist-support
      └── 01 listing → 02 store → 03a packageName → 03b prompt → 04 orchestrator → 05 CLI
          (this PR: 📍 1)