Implement task 032: sound system

Add pkg/audio with SoundSystem backed by WAV files extracted from the
disassembly project (44100 Hz, 16-bit PCM mono, embedded via embed.FS).

Continuous sounds (engine ×3, low-fuel warning) loop via
audio.NewInfiniteLoop; the engine player swaps immediately on speed
change. The refuel beep is a one-shot replayed each frame while over a
depot (spec §14.5). All other sounds are one-shot edge-triggered from
ControlFlags (fire, explosion, bonus life, fuel-full) or state
transitions (shell whistle on IsFlying, heli missile on Active).

All sounds are suppressed during scroll-in and while paused (spec §14.7);
StopAll is called on those frames and also on restart / return-to-menu.

Supporting changes:
- ControlFlags.FuelFull: set once when fuel transitions to cap during
  refueling (FuelResult.FuelFull from UpdateFuel).
- Controls.FireSound: set in applyInput when a missile actually launches.
- go.mod/go.sum: add ebitengine/oto/v3 (pulled in by ebiten/v2/audio).

Author: morozov

go.mod

@@ -60,6 +60,7 @@ require (
 	github.com/dlclark/regexp2 v1.11.5 // indirect
 	github.com/ebitengine/gomobile v0.0.0-20250923094054-ea854a63cce1 // indirect
 	github.com/ebitengine/hideconsole v1.0.0 // indirect
+	github.com/ebitengine/oto/v3 v3.4.0 // indirect
 	github.com/ebitengine/purego v0.9.1 // indirect
 	github.com/ettle/strcase v0.2.0 // indirect
 	github.com/fatih/color v1.18.0 // indirect

go.sum

@@ -160,6 +160,8 @@ github.com/ebitengine/gomobile v0.0.0-20250923094054-ea854a63cce1 h1:+kz5iTT3L7u
 github.com/ebitengine/gomobile v0.0.0-20250923094054-ea854a63cce1/go.mod h1:lKJoeixeJwnFmYsBny4vvCJGVFc3aYDalhuDsfZzWHI=
 github.com/ebitengine/hideconsole v1.0.0 h1:5J4U0kXF+pv/DhiXt5/lTz0eO5ogJ1iXb8Yj1yReDqE=
 github.com/ebitengine/hideconsole v1.0.0/go.mod h1:hTTBTvVYWKBuxPr7peweneWdkUwEuHuB3C1R/ielR1A=
+github.com/ebitengine/oto/v3 v3.4.0 h1:br0PgASsEWaoWn38b2Goe7m1GKFYfNgnsjSd5Gg+/bQ=
+github.com/ebitengine/oto/v3 v3.4.0/go.mod h1:IOleLVD0m+CMak3mRVwsYY8vTctQgOM0iiL6S7Ar7eI=
 github.com/ebitengine/purego v0.9.1 h1:a/k2f2HQU3Pi399RPW1MOaZyhKJL9w/xFpKAg4q1s0A=
 github.com/ebitengine/purego v0.9.1/go.mod h1:iIjxzd6CiRiOG0UyXP+V1+jWqUXVjPKLAI0mRfJZTmQ=
 github.com/envoyproxy/go-control-plane v0.9.0/go.mod h1:YTl/9mNaCwkRvm6d1a2C3ymFceY/DCBVvsKhRF0iEA4=