Apply perceptual curve to volume sliders (#4272)

## Problem

Players reported having to turn the volume slider down to ~20% before
noticing any change in loudness.

The sliders fed their linear 0–1 position straight to Howler's
`volume()`, which is linear amplitude gain. Human loudness perception is
roughly logarithmic, so the top ~80% of the slider all sounds nearly
identical — the classic linear-fader problem.

## Fix

Square the slider position into a perceptual (audio-taper) gain inside
`SoundManager`. The stored setting and the displayed `%` remain the
intuitive linear slider position; only the gain handed to Howler is
curved.

| Slider | Old gain (linear) | New gain (x²) |
|--------|-------------------|---------------|
| 100%   | 1.00              | 1.00          |
| 90%    | 0.90              | 0.81          |
| 80%    | 0.80              | 0.64          |
| 50%    | 0.50              | 0.25          |
| 20%    | 0.20              | 0.04          |

Lowering the slider from 100→80 now produces an audible drop instead of
nothing until ~20%.

## Notes

- Quadratic (x²) was chosen as a balanced, conservative taper. Cubic
(x³) would make the top-end drop-off even more immediate if preferred.
- Existing saved settings are unaffected; the same slider position will
simply sound slightly quieter, which is the intended correction.

## Tests

Updated `SoundManager.test.ts` to assert the curved gain and added a
dedicated test locking in the top-of-range behavior. All 18 tests pass.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/client/sound/SoundManager.test.ts

@@ -113,7 +113,8 @@ describe("SoundManager", () => {
     new SoundManager(bus, settings);
     const bgHowls = howlInstances.slice(0, 3);
     bgHowls.forEach((h) => {
-      expect(h.volume).toHaveBeenCalledWith(0.5);
+      // Slider position is curved (squared) into perceptual gain: 0.5² = 0.25.
+      expect(h.volume).toHaveBeenCalledWith(0.25);
     });
   });
 
@@ -124,8 +125,9 @@ describe("SoundManager", () => {
     howlInstances.length = 0;
     new SoundManager(bus, settings);
     bus.emit(new PlaySoundEffectEvent("click"));
+    // Slider position 0.3 is curved (squared) into perceptual gain: 0.3² = 0.09.
     expect(howlCtor).toHaveBeenLastCalledWith(
-      expect.objectContaining({ volume: 0.3 }),
+      expect.objectContaining({ volume: 0.09 }),
     );
   });
 
@@ -133,7 +135,8 @@ describe("SoundManager", () => {
     eventBus.emit(new SetBackgroundMusicVolumeEvent(0.7));
     const bgHowls = howlInstances.slice(0, 3);
     bgHowls.forEach((h) => {
-      expect(h.volume).toHaveBeenCalledWith(0.7);
+      // 0.7² = 0.49 perceptual gain.
+      expect(h.volume).toHaveBeenCalledWith(0.7 * 0.7);
     });
   });
 
@@ -142,7 +145,8 @@ describe("SoundManager", () => {
     const clickHowl = howlInstances[howlInstances.length - 1];
     clickHowl.volume.mockClear();
     eventBus.emit(new SetSoundEffectsVolumeEvent(0.4));
-    expect(clickHowl.volume).toHaveBeenCalledWith(0.4);
+    // 0.4² = 0.16 perceptual gain.
+    expect(clickHowl.volume).toHaveBeenCalledWith(0.4 * 0.4);
   });
 
   it("clamps volume values between 0 and 1", () => {
@@ -159,6 +163,16 @@ describe("SoundManager", () => {
     });
   });
 
+  it("curves the slider position into perceptual gain so the top of the range is audibly distinct", () => {
+    const bgHowls = howlInstances.slice(0, 3);
+    // Linear gain would make 0.9 and 1.0 nearly indistinguishable; squaring
+    // spreads the top end (0.9 → 0.81) so reductions are noticeable sooner.
+    eventBus.emit(new SetBackgroundMusicVolumeEvent(0.9));
+    bgHowls.forEach((h) => {
+      expect(h.volume).toHaveBeenLastCalledWith(0.81);
+    });
+  });
+
   it("dispose() unsubscribes from EventBus so events no longer play sounds", () => {
     eventBus.emit(new PlaySoundEffectEvent("click"));
     const clickHowl = howlInstances[howlInstances.length - 1];