docs: fold shipped specs into the architecture docs and delete them

A spec was a promise; sixteen of them had become descriptions of code that
already shipped, sitting beside four that describe work still outstanding, with
nothing in the file telling the two apart. Half the statuses were also wrong —
audio-equalizer read "Accepted" with the EQ live on both platforms, the native
video spec said the flag stays off after the default was flipped on.

The shipped designs move into docs/architecture, which is the maintained
description of the build, and the spec files go. Git history keeps the
originals; what a future change still needs is carried across:

- 01-rust-backend: favourites rewritten (the old section named a file that no
  longer exists and called shipped buttons "planned"), domain vocabulary owned
  by Rust (SearchScope, exclusions, the bitrate ladder), background workers
- 02-svelte-frontend: app shell and chrome, library mosaic, series/episode
  navigation, downloaded browse, safe-area insets, native-video store, logging
- 03-data-flow: locally-indexed search
- 05-platform-backends: audio settings on ExoPlayer, the equalizer's band
  vocabulary, native video compositing, the background-audio handoff
- 06-downloads-and-offline: one storage model, offline catalog visibility
- 09-security: path confinement and input binding

docs/specs/README.md now says what the directory is for and where each shipped
design went. Deferred work the specs recorded is kept beside the code it
concerns rather than lost: season-bounded autoplay, the two dead search
commands, why indexing is a full crawl.

requirements.md had fourteen stale statuses — Android audio parity still read
"Linux only", DR-150 still said the native-video default was off, DR-190 was
Proposed after DR-196 implemented it, and five tooling requirements were
Proposed after landing. Three unbuilt specs suggested requirement ids that have
since been allocated to other work; each now carries a warning.

docs/specs/libmpv2-migration.md

@@ -1,6 +1,8 @@
 # Spec: Migrate to libmpv2 and declare the project licence
 
-**Status:** Proposed
+**Status:** Partially implemented — the `LICENSE` file has landed (part 2). The
+`libmpv` → `libmpv2` swap (part 1) is **not** done: `src-tauri/Cargo.toml` still
+pins the abandoned crate to a git branch.
 **Requirements:** UR-003 → IR-003 (revises the MPV integration); no new user-facing behaviour
 **UX spec:** n/a
 **Supersedes / revises:** dependency and licensing housekeeping identified in [playback-backend-unification.md](playback-backend-unification.md)