[Linuxuser67]

Cleanup with zero behavior delta (all pre-existing argv tests pass unmodified):

• Drop StreamSel.headers: write-only since parts moved to the binary (verified zero readers). The HttpHeaders type stays for the live HLS path.
• New ytdlp_identity_args helper (cookies + optional UA + end-of-options separator + page URL) replacing three hand-assembled copies in extraction, part and HLS spawns. Extraction keeps its exact behavior (no UA there — None); the live-resolve branch adopts it on rebase.

214 passing, clippy clean. Disjoint hunks from PR #46 (only shared context is the helper it should adopt).