docs: update AGENTS.md and README for new features, add JS tests

Docs:
- AGENTS.md: updated toolchain section with js-lint/js-test tasks,
  expanded web/ui/ directory listing, added event delegation and
  Web Audio API to key patterns, updated testing section
- README: marked issue #9 (Audio Preview Player) as done
- .gitignore: added akai-fetch.test and cover.out

Tests:
- Moved groupWavs and cleanName to util.js (pure functions)
- Added 10 new JS tests: 6 groupWavs, 4 cleanName
- Total JS tests: 29 (up from 19)

README.md

@@ -110,7 +110,7 @@ See [open issues](https://git.notsosm.art/david/akai-utils/issues).
 | #6 | Direct-to-Disk Take Extraction | todo |
 | #7 | TAR Bulk Import/Export | todo |
 | #8 | Sample Tag Editor | todo |
-| #9 | Audio Preview Player | todo |
+| #9 | Audio Preview Player | done |
 
 ## License