feat: separate music and sound volumes

[LucasAntunesdeAlmeida]

What

• g.MusicVolume(v float64): the music's own volume, 0..1 (default 1).
• g.SoundVolume(v float64): the sound effects' own volume, 0..1 (default 1).
• g.Volume(v) stays the master and scales both.
• README cheat-sheet rows; horde README section.

Design

• Multiplied with the master. A player's level is channel volume times master volume. A mute key (Volume(0)) still silences everything, and unmuting restores each channel where the settings menu left it, with no bookkeeping in the game.
• Same contract as Volume. Clamped to 0..1, NaN counts as 0, callable any time. Volume and MusicVolume update the playing music at once; SoundVolume applies to sounds started afterwards (fire-and-forget effects already playing finish at their old volume, as with Volume).
• Zero skips work. A sound whose level is 0 (either volume at 0) now returns before any work: no file read, no decode, no player (before, a muted sound still decoded and cached its PCM). Music still starts silently at level 0 so raising the volume brings it back in place, as before.
• Storage. assets.Cache keeps the three volumes in one array indexed by an internal Channel (Master, Music, Sounds) under its existing mutex; Level(ch) computes the effective value. Internal only: the public API is the three setters. No getters, as with Volume: a game's settings screen owns its values (and persists them with g.Save).
• Headless never touches audio, unchanged: the setters only store values there.

Proof

• volume_test.go (no audio device): each setter's default 1 and clamping (0.5, -1, 2, 0, 1, NaN, +Inf); levels multiply (0.5 master x 0.4 music = 0.2, x 0.8 sounds = 0.4) and channels keep their value under a master change; muting sounds leaves the music; a windowed game's Sound on a missing file does nothing with SoundVolume(0) or Volume(0) and panics (tries to load) once audible, proving muted sounds do no work; headless games can call all three from OnUpdate.
• examples/horde: the pause overlay gains "4 MUSIC 100%" and "5 SOUNDS 100%" (4/5 keys, LB/RB on a pad with pad prompts, or a click), cycling 100, 50, 0%; a new sound level plays a sample hit. M still mutes everything on top.
• Driven over MCP headless: 4 cycles the music label 100, 50, 0% (holding the key does not repeat), RB and a click cycle the sounds, and the labels switch to LB/RB once a pad is used.
• Local gate: gofmt, vet, staticcheck, tests, js/wasm vet + build.