docs(design): commit music-config-without-emms spec + readiness review

The spec lays out the EMMS-removal design: package-owned track and
playlist structs, a narrow backend protocol with mpv as the v1 backend,
state-change hooks replacing EMMS player hooks, an overlay-based
selected-track marker, a fake-backend test architecture, a quantified
performance budget, a 22-step parity walk, and the migration plan.

The review tracks implementation readiness: which migration-plan step
is safe to start, which decisions still block the rest, and the exact
spec edits required.

Two decisions landed this session and are now baked into the spec:

- Platform support: Linux and macOS get full features; Windows runs in
  degraded mode (play/stop/next/previous only) because Emacs cannot
  natively connect to mpv's Windows named-pipe IPC.  Anyone who wants
  full Windows parity can wire mpvc.exe shellout or a w32-* named-pipe
  client as a follow-up.

- File-extension scope: cj/music-file-extensions stays as-is.  webm and
  ape files in ~/music are intentionally skipped.

Socket path now references temporary-file-directory instead of a
hardcoded /tmp/ prefix so the spec stays consistent with the Windows
section.

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