Fix refuel sound: one-shot per game frame, not a looping sound

Refuel plays once per game loop iteration (~12 FPS), producing distinct
audible beeps ~83 ms apart. Looping a 20 ms buffer at 50 Hz would
collapse them into a continuous tone. Remove from the looping table and
fix the Loop? flag in the asset catalog.

Author: morozov

spec/14-sound.md

@@ -151,7 +151,6 @@ playback rate.
 | Sound | Buffer | Loop rate | `Update()` responsibility |
 |---|---|---|---|
 | Engine (×3 variants) | 20 ms | 50 Hz | Start when active, stop when inactive, swap player on speed change |
-| Refuel | 20 ms | 50 Hz | Start when refueling, stop when done |
 | Low-fuel warning | 848 ms | ~1.18 Hz | Start when flag set, stop when cleared |
 
 ### Speed Variant Switching
@@ -187,7 +186,7 @@ The WAV files are written to `html/river-raid/audio/`:
 | `engine-normal.wav` | Engine at normal speed | Yes |
 | `engine-fast.wav` | Engine at fast speed | Yes |
 | `engine-slow.wav` | Engine at slow speed | Yes |
-| `refuel.wav` | Refueling beep | Yes |
+| `refuel.wav` | Refueling beep | No |
 | `low-fuel.wav` | Low fuel warning warble | Yes |
 | `fuel-full.wav` | Tank full beep | No |
 | `fire.wav` | Missile fire burst | No |
@@ -224,7 +223,20 @@ func loadLoopingSound(ctx *audio.Context, name string) *audio.Player {
 
 Use `audio.Context` with a sample rate of **44100** to match the WAV files.
 
-## 14.7. Implementation Notes
+## 14.7. Sound Suppression
+
+Sound is suppressed (all players stopped) in two gameplay sub-states:
+
+- **Scroll-in** (`GameplayScrollIn`): no engine or other sounds while the terrain
+  is scrolling into view before the player takes control.
+- **Paused** (`GameState.Paused == true`): all sound stops while the game is
+  paused and resumes naturally when the player unpauses.
+
+In both cases `SoundSystem.Update` detects the suppressed condition, stops all
+active players, and resets its edge-detection state. Sounds resume automatically
+on the next `Update` call that finds the game in a normal playable state.
+
+## 14.8. Implementation Notes
 
 - Pre-compute all sound waveforms as PCM samples at initialization time
 - Engine sounds loop seamlessly while the corresponding speed is active