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/crypt.json

@@ -0,0 +1,12 @@
+{
+  "name": "crypt",
+  "about": "Dark ambient for the small hours — occult dread, deep focus.",
+  "license": "CC BY 4.0 — Kevin MacLeod (incompetech.com)",
+  "tracks": [
+    { "title": "Ossuary 5 - Rest", "artist": "Kevin MacLeod", "src": "crypt/01-ossuary-5-rest.mp3", "secs": 235 },
+    { "title": "Ghostpocalypse - Crossing the Threshold", "artist": "Kevin MacLeod", "src": "crypt/02-ghostpocalypse-crossing-the-threshold.mp3", "secs": 108 },
+    { "title": "Crypto", "artist": "Kevin MacLeod", "src": "crypt/03-crypto.mp3", "secs": 204 },
+    { "title": "Killers", "artist": "Kevin MacLeod", "src": "crypt/04-killers.mp3", "secs": 305 },
+    { "title": "Hitman", "artist": "Kevin MacLeod", "src": "crypt/05-hitman.mp3", "secs": 200 }
+  ]
+}