feat(music): bundled CC-BY session soundtrack + /music player

Add a session background-music feature to the TUI. Ships two bundled
CC BY 4.0 albums (Kevin MacLeod / incompetech.com) under hh/music/:
'crypt' (dark ambient, 5 tracks) and 'terminal' (synth/chiptune, 6).

- hh/src/music.rs: playlist model + subprocess player (ffplay/mpv/cvlc
  fallback chain), album discovery (user ~/.hh/music shadows bundled),
  random shuffle, and /music import of the operator's own audio.
- /music [list] · play [album] (blank/random shuffles) · stop · next ·
  import <path> [as <name>]; now-playing shown in the top bar + help.
- docs/music-licensing.md: CC-BY provenance/attribution register.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hh/music/terminal.json

@@ -0,0 +1,13 @@
+{
+  "name": "terminal",
+  "about": "Hacker synth & chiptune — neon arcades and late-night shells.",
+  "license": "CC BY 4.0 — Kevin MacLeod (incompetech.com)",
+  "tracks": [
+    { "title": "Neon Laser Horizon", "artist": "Kevin MacLeod", "src": "terminal/01-neon-laser-horizon.mp3", "secs": 178 },
+    { "title": "Cyborg Ninja", "artist": "Kevin MacLeod", "src": "terminal/02-cyborg-ninja.mp3", "secs": 180 },
+    { "title": "Space Fighter Loop", "artist": "Kevin MacLeod", "src": "terminal/03-space-fighter-loop.mp3", "secs": 101 },
+    { "title": "Voltaic", "artist": "Kevin MacLeod", "src": "terminal/04-voltaic.mp3", "secs": 196 },
+    { "title": "Blip Stream", "artist": "Kevin MacLeod", "src": "terminal/05-blip-stream.mp3", "secs": 284 },
+    { "title": "Pixelland", "artist": "Kevin MacLeod", "src": "terminal/06-pixelland.mp3", "secs": 233 }
+  ]
+}