refactor(server): extract the highway WebSocket into routers/ws_highway.py (R3) (#844)

The single largest handler in server.py — the 902-line /ws/highway/{filename}
chart streamer — plus its 3 exclusive helpers (_pick_smart_arrangement,
_sanitize_authors, _sanitized_song_offset). server.py: 9,008 -> 8,003 (-1,005,
the biggest single R3 cut).

Clean move despite the size: the handler's only server-module deps are the 4
path constants + 3 exclusive helpers + app + log. Everything else it uses is
either NESTED inside the handler (_evict_audio_cache, _fill_scale_degree,
_manifest_entries, _tone_names, _xml_rank, _send_keepalives) or imported from
the shared lib modules (song/audio/sloppak/drums/notation/dlc_paths/metadata_db).

- Path constants read through the appstate seam: added static_dir /
  sloppak_cache_dir / audio_cache_dir slots (config_dir already there);
  server.py configures them. Bodies otherwise verbatim (@app.websocket ->
  @router.websocket, PATHS -> appstate.*, log -> module logger).
- sloppak_cache_dir IS setattr-patched, so the 3 test_highway_ws_* suites now
  also `setattr(appstate, "sloppak_cache_dir", ...)` next to their existing
  server patch. _sanitize_authors unit tests import it from routers.ws_highway
  (it moved). No other test churn.
- Removed 23 now-dead imports from server.py (song/audio/drums/notation/
  bisect/contextvars/structlog/WebSocket*/_arr_smart_sort_key) — diffed against
  the origin/main unused-import baseline so only NEWLY-dead ones went.

owns_tmp (assigned, never read) moved verbatim — it's pre-existing dead on
origin/main too; left as-is to keep the move faithful.

Verified: route table identical to origin/main (143, paths/methods/order);
handler body verbatim spot-checked; pyflakes clean (server has no new
undefined/dead); pytest 2400 passed; packaging guard 53; eslint 0. Boot smoke:
the highway WS streams the full chart (song_info/beats/sections/notes/chords/
notation/anchors/drum_tab -> ready) BYTE-for-byte the same message sequence as
origin/main across arrangements 0/1/2, zero tracebacks.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -26,7 +26,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   next root-level module can't ship broken.
 
 ### Added
-- **`routers/` — extracting `server.py`'s route layer, cheapest-first (R3).** Each PR moves a cohesive route group into a `fastapi.APIRouter` under `lib/routers/`, mounted with `app.include_router(...)` at its original site (FastAPI matches in registration order; the full route table stays byte-identical). Bodies are verbatim — only the decorator receiver (`@app` → `@router`) and singleton reads (`meta_db` → `appstate.meta_db`, resolved at call time) change. So far: `audio_effects` (5), `artist_aliases` (5), `loops` (3), `playlists` (12 + custom covers). The DLC library-path resolution (`_get_dlc_dir`, pure `_resolve_dlc_path`) moved to `lib/dlc_paths.py`, reading paths through the seam; `config_dir`/`dlc_dir`/`dlc_dir_env` now ride the `appstate` seam (env-derived, so the pop-and-reimport fixtures reconfigure it for free), and the shared request-field sanitizer `_clean_str` moved to `lib/reqfields.py`. The next cut is picked by a dependency-closure scan that ranks groups by how many `monkeypatch.setattr(server, …)` targets they'd drag along.
+- **`routers/` — extracting `server.py`'s route layer, cheapest-first (R3).** Each PR moves a cohesive route group into a `fastapi.APIRouter` under `lib/routers/`, mounted with `app.include_router(...)` at its original site (FastAPI matches in registration order; the full route table stays byte-identical). Bodies are verbatim — only the decorator receiver (`@app` → `@router`) and singleton reads (`meta_db` → `appstate.meta_db`, resolved at call time) change. So far: `audio_effects` (5), `artist_aliases` (5), `loops` (3), `playlists` (12 + covers), `ws_highway` (the 902-line highway chart WebSocket — the single largest handler). The DLC library-path resolution (`_get_dlc_dir`, pure `_resolve_dlc_path`) moved to `lib/dlc_paths.py`, reading paths through the seam; `config_dir`/`dlc_dir`/`dlc_dir_env` now ride the `appstate` seam (env-derived, so the pop-and-reimport fixtures reconfigure it for free), and the shared request-field sanitizer `_clean_str` moved to `lib/reqfields.py`. The next cut is picked by a dependency-closure scan that ranks groups by how many `monkeypatch.setattr(server, …)` targets they'd drag along.
 - **`routers/` — the first extracted route module (R3).** The five audio-effects mapping
   endpoints move out of `server.py` into `lib/routers/audio_effects.py` as a
   `fastapi.APIRouter`, mounted with `app.include_router(...)` **at the point in the file