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>

docs/music-licensing.md

@@ -0,0 +1,53 @@
+# Session-music licensing & provenance
+
+_Generated 2026-07-07 for the bundled albums under `hh/music/`._
+
+This is the provenance + attribution register for the background-music albums that
+the Rust client plays during a session (`/music`, see `hh/src/music.rs`). Each album
+is a `hh/music/<name>.json` manifest pointing at the `.mp3` files shipped alongside it.
+
+## Verdict: ALL bundled tracks CLEARED — CC BY 4.0, attribution required
+
+- **2 albums / 11 tracks**, every one composed by **Kevin MacLeod** and published on
+  **incompetech.com** under **Creative Commons Attribution 4.0 International (CC BY 4.0)**.
+- CC BY 4.0 **permits** redistribution, bundling, and commercial use **provided the
+  author is credited**. That makes these safe to commit to the repo, push to any
+  remote, demo publicly, and ship — unlike the quarantined character art
+  (`character-art-licensing.md`).
+- Attribution is carried in two machine-readable places already: the `license` field
+  of each `hh/music/*.json` manifest, and this register. Keep both intact.
+
+### Required attribution (CC BY 4.0)
+
+> Music by **Kevin MacLeod** (incompetech.com) — licensed under
+> **Creative Commons: By Attribution 4.0** — https://creativecommons.org/licenses/by/4.0/
+
+Surface this credit wherever the music is used publicly (demo video description,
+release notes, an About/Credits screen). Do not remove the `license` fields from the
+manifests, and do not relabel these tracks as original or royalty-free-without-credit.
+
+## Per-track register
+
+| Album | Track | Composer | Source | Licence | Status |
+|-------|-------|----------|--------|---------|--------|
+| crypt | Ossuary 5 - Rest | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+| crypt | Ghostpocalypse - 6 Crossing the Threshold | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+| crypt | Crypto | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+| crypt | Killers | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+| crypt | Hitman | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+| terminal | Neon Laser Horizon | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+| terminal | Cyborg Ninja | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+| terminal | Space Fighter Loop | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+| terminal | Voltaic | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+| terminal | Blip Stream | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+| terminal | Pixelland | Kevin MacLeod | incompetech.com | CC BY 4.0 | CLEARED |
+
+> `crypt` is the dark-ambient album (matches the default "crypt" vestment); `terminal`
+> is the hacker synth/chiptune album. Files live under `hh/music/<album>/`.
+
+## Imported albums (`/music import`)
+
+`/music import <path>` writes user albums to `~/.hh/music/*.json`, referencing the
+operator's own files **in place** (absolute paths — nothing is copied into the repo).
+Those files are the operator's responsibility and are **out of scope** for this
+register; only the bundled `hh/music/` albums above are shipped and covered here.