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>

tests/test_highway_ws_notation.py

@@ -123,6 +123,9 @@ def make_client(tmp_path, monkeypatch):
         monkeypatch.setattr(server, "load_plugins", lambda *a, **kw: None)
         monkeypatch.setattr(server, "startup_scan", lambda: None)
         monkeypatch.setattr(server, "SLOPPAK_CACHE_DIR", tmp_path / "cache")
+        # ws_highway reads the cache dir through the appstate seam now.
+        import appstate as _appstate
+        monkeypatch.setattr(_appstate, "sloppak_cache_dir", tmp_path / "cache")
         return server
 
     (tmp_path / "dlc").mkdir()