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docs(specs): add backend-owned stream selection

Rust becomes the single owner of which stream to play — direct play or
transcode, at what ceiling, over what transport — and hands every player
backend a self-describing StreamSelection instead of a bare URL. mpv,
ExoPlayer and the HTML5/hls.js path all consume one decision rather than
three places re-deriving it.

The motivating leak is concrete. VideoPlayer.svelte determines transport with
`currentStreamUrl.includes(".m3u8")`, in two places, for a URL Rust
constructed and therefore already knows the shape of. That is the boundary
rule in miniature: not item-type taxonomy, but the same error of
reconstructing a domain fact in the presentation layer because the wire shape
did not carry it. A tagged Transport enum deletes it.

The design line, which ExoPlayer forces: Rust decides *what stream*, the
player decides *how to deliver it*. ExoPlayer has genuine adaptive track
selection; this spec must not reimplement or fight it. Rust only adapts where
the player cannot (mpv) and the server actually offers a ladder.

Six phases, and phase 1 stands alone as pure ownership movement with no
behaviour change. Phase 4 (direct-play negotiation) is what removes the
transcode and unblocks the Linux native-video work. Phase 5 (adaptation) is
gated on counting EXT-X-STREAM-INF entries in a real playlist — the
acceptance criteria require that count be recorded before it is either
started or dropped.

Takes DR-121 from read-through-media-cache.md, which specced Rust-owned
quality reporting but never built it; that spec keeps its capture half.

2026-08-21 19:41:32 +02:00