fix(bass-presets): move factory presets to channel subdirs + per-channel install

Root cause: factory presets for bass (banks 17-20) sat in the root
factory dir alongside guitar presets, and install_factory_presets()
used rglob() which picked up ALL presets into the GUITAR channel.

Changes:
- Move bank_17-20 from presets/factory/ → presets/factory/bass/
  (matching the existing keys/, vocals/, backing_tracks/ layout)
- Fix install_factory_presets() to scan channel-specific subdirectory:
  GUITAR scans root bank_* dirs, all others scan <channel>/bank_*
- Fix main.py to iterate over all 5 channels when installing factory
  presets (was only installing into GUITAR)
- Import Channel enum in main.py

Tests: 56/56 preset manager tests pass. Integration verified:
- GUITAR: 68 presets (banks 0-16)
- BASS: 16 presets (banks 17-20, SansAmp/Darkglass/Ampeg SVT)
- KEYS: 12 presets (banks 21-23)
- VOCALS: 12 presets (banks 24-26)
- BACKING_TRACKS: 12 presets (banks 27-29)

docs/nam-ir-inventory.md

@@ -382,6 +382,11 @@ The following Archive.org sources returned 404 when last checked (2026-06-13):
 │   ├── Orange_Rocker30_Dirty_A2.nam       # 295 KB — Rock crunch
 │   ├── Orange_Rocker30_Natural_A2.nam     # 289 KB — Clean edge
 │   ├── Fender_Bassman_Dimed_A2.nam        # 289 KB — Blues/rock breakup
+│   │
+│   └── 🎸 Bass Genre Research:
+│       └── Full artist tone chain configs → `docs/bass-tone-research.md`
+│           Available on Tone3000 (needs download): SansAmp BDDI v1, Darkglass B3K/B7K,
+│           Ampeg SVT-CL, Geddy Lee SansAmp DI-2112
 │   ├── Hiwatt_DR103_Bright_A2.nam         # 287 KB — Classic rock
 │   ├── Hiwatt_DR103_Cornish_A2.nam        # 288 KB — Boosted classic rock
 │   ├── Supro_Black_Magick_M3_A2.nam       # 288 KB — Garage/blues