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.

src-tauri/src/commands/catalog.rs

@@ -319,7 +319,8 @@ async fn read_last_sync(
 ///
 /// Replaces the frontend's startup-only `syncCatalog()` call: index freshness is
 /// sync policy and belongs in Rust (see the layer assignment in
-/// docs/specs/catalog-index-search.md). Ticks every [`CATALOG_INDEX_TICK`] and
+/// docs/architecture/03-data-flow.md, "Search Flow"). Ticks every
+/// [`CATALOG_INDEX_TICK`] and
 /// runs a pass when a repository exists, the server is reachable, and the index
 /// is older than [`CATALOG_INDEX_TTL`].
 ///

src-tauri/src/player/mpv_backend.rs

@@ -630,7 +630,7 @@ impl PlayerBackend for MpvBackend {
         // peaking bands and (optionally) a dynamic loudness normalizer, and
         // set the `af` property. An empty string clears all filters. Both
         // features share one `af` graph because MPV exposes a single filter
-        // property. See docs/specs/audio-equalizer.md and IR-020.
+        // property. See docs/architecture/05-platform-backends.md and IR-020.
         let af = build_af_filter(settings);
         self.mpv
             .set_property("af", af.as_str())

src-tauri/src/settings.rs

@@ -28,7 +28,9 @@ impl VolumeLevel {
 
 /// Centre frequencies (Hz) of the fixed 10-band ISO equalizer. The band count
 /// and layout are a property of the audio engine, not the UI — presets and the
-/// MPV filter are defined against these bands. See docs/specs/audio-equalizer.md.
+/// MPV filter are defined against these bands. See
+/// docs/architecture/05-platform-backends.md ("The equalizer, and where its
+/// vocabulary lives").
 ///
 /// TRACES: UR-027 | DR-030, IR-020
 pub const EQ_BANDS: [f32; 10] = [
@@ -159,7 +161,8 @@ impl AudioSettings {
 /// The ladder is deliberately expressed in bandwidth rather than resolution: it
 /// exists to fit a connection, and the resolution cap is chosen *from* the
 /// bitrate so the encoder does not spend a small budget on pixels it cannot
-/// afford. See docs/specs/streaming-bitrate-cap.md.
+/// afford. See docs/architecture/01-rust-backend.md ("Streaming quality
+/// ladder").
 ///
 /// TRACES: UR-074 | DR-162
 #[derive(specta::Type, Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]