feat: add band_extract audio source type for frequency band filtering

New audio source type that filters a parent source to a specific frequency
band (bass 20-250Hz, mid 250-4kHz, treble 4k-20kHz, or custom range).
Supports chaining with frequency range intersection and cycle detection.
Band filtering applied in both CSS audio streams and test WebSocket.

TODO-css-improvements.md

@@ -24,7 +24,7 @@
 
 ## New Source Types
 
-- [ ] **`weather`** — Weather-reactive ambient: maps weather conditions (rain, snow, clear, storm) to colors/animations via API
+- [x] **`weather`** — Weather-reactive ambient: maps weather conditions (rain, snow, clear, storm) to colors/animations via API
 - [ ] **`music_sync`** — Beat-synced patterns: BPM detection, energy envelope, drop detection (higher-level than raw `audio`)
 - [ ] **`math_wave`** — Mathematical wave generator: user-defined sine/triangle/sawtooth expressions, superposition
 - [ ] **`text_scroll`** — Scrolling text marquee: bitmap font rendering, static text or RSS/API data source *(delayed)*
@@ -53,7 +53,7 @@ Need to research HAOS communication options first (WebSocket API, REST API, MQTT
 
 ### `audio`
 
-- [ ] New audio source type: band extractor (bass/mid/treble split) — responsibility of audio source layer, not CSS
+- [x] New audio source type: band extractor (bass/mid/treble split) — responsibility of audio source layer, not CSS
 - [ ] Peak hold indicator: global option on audio source (not per-mode), configurable decay time
 
 ### `daylight`