docs(specs): restore the stream-selection spec dropped by the squash

CI caught this, which is what it is for: `check-doc-links.sh` failed because
media-player-controller.md links to backend-owned-stream-selection.md twice and
the file was not on master.

My fault, and worth recording how. That spec was written and committed directly
on master early on, before the work moved into a worktree. Squashing the branch
began with `git reset --hard` back to the merge base, which dropped those early
master-only commits — and the squash then brought in a document referencing one
of them.

Nothing was lost: the commits are still reachable, and the file is restored from
1d56517f along with its index entries in docs/specs/README.md and the docs site.

The lesson is narrower than "be careful with reset": a squash whose base is
chosen by hand silently drops anything committed outside the branch being
squashed. Only the link checker noticed, because it is the one gate that reads
across files rather than within them — and it is also the one gate I had not
run locally before pushing.

All CI checks now pass locally: boundary, doc links, tooling, format, lint at
the 158 ceiling, traces validate, coverage 90%.

docs-site/SUMMARY.md

@@ -33,6 +33,7 @@
 - [Spec Review Checklist](specs/SPEC-REVIEW-CHECKLIST.md)
 - [Playback Backend Unification](specs/playback-backend-unification.md)
 - [Linux Native Video Spike](specs/linux-native-video-spike.md)
+- [Backend-Owned Stream Selection](specs/backend-owned-stream-selection.md)
 - [Player Facade Enforcement](specs/player-facade-enforcement.md)
 - [Windows Native Audio Backend](specs/windows-native-audio-backend.md)
 - [libmpv2 Migration](specs/libmpv2-migration.md)