fix(audio): smooth, monotonic volume curve (#84)

applyVolume() mapped 0-100 with a two-piece, discontinuous curve: gain =
(vol/100)*0.2 for vol<100 (so the whole 0-99 range only spanned 0..0.198, making
80->99 feel flat) then a raw passthrough at vol===100 (a ~5x jump to full
loudness). That produced the reported dead zone + sudden ear-blast at 100.

Replace it with a single continuous, strictly-monotonic curve
volumeToFactor(v) = 0.2*x + 0.8*x^8 (x = v/100): 0 at 0, exactly 1.0 at 100, no
flat region and no discontinuity, so the slider feels proportional and full
loudness is still reserved at 100. Extracted as an exported pure function and
unit-tested (boundaries, strict monotonicity, dead-zone removal, no jump at 100).

Note: per the maintainer's note on #84 the >80% suppression was intentional
ear-protection; this change makes the upper range (above ~75%) audibly louder
than before in exchange for a proportional slider — applied per maintainer
decision.

Fixes #84

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/audio/player.test.ts

@@ -2,7 +2,7 @@ import { describe, it, expect } from "vitest";
 import { mkdtempSync, writeFileSync, existsSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
-import { buildFfmpegArgs, shouldUsePowerShellDownload, cleanupTempDir } from "./player.js";
+import { buildFfmpegArgs, shouldUsePowerShellDownload, cleanupTempDir, volumeToFactor } from "./player.js";
 
 function getHeadersArg(args: string[]): string {
   const idx = args.indexOf("-headers");
@@ -80,6 +80,37 @@ describe("buildFfmpegArgs", () => {
   });
 });
 
+describe("volumeToFactor (#84 smooth volume curve)", () => {
+  it("is 0 at vol 0 and exactly 1.0 at vol 100 (full loudness still reserved at 100)", () => {
+    expect(volumeToFactor(0)).toBe(0);
+    expect(volumeToFactor(100)).toBe(1);
+  });
+
+  it("clamps out-of-range input", () => {
+    expect(volumeToFactor(-20)).toBe(0);
+    expect(volumeToFactor(150)).toBe(1);
+  });
+
+  it("is strictly monotonic across the whole range (no dead zone)", () => {
+    for (let v = 0; v < 100; v++) {
+      expect(volumeToFactor(v + 1)).toBeGreaterThan(volumeToFactor(v));
+    }
+  });
+
+  it("removes the old flat 80-99 dead zone", () => {
+    // Old mapping moved only 0.16 -> 0.198 across 80..99; new curve climbs clearly.
+    expect(volumeToFactor(99) - volumeToFactor(80)).toBeGreaterThan(0.3);
+  });
+
+  it("removes the discontinuity at 100 (old jump was ~0.8)", () => {
+    expect(volumeToFactor(100) - volumeToFactor(99)).toBeLessThan(0.1);
+  });
+
+  it("keeps the low range gentle", () => {
+    expect(volumeToFactor(50)).toBeLessThan(0.12);
+  });
+});
+
 describe("shouldUsePowerShellDownload", () => {
   const jdymusicUrl =
     "http://m801.music.126.net/20260507/abc/jdymusic/obj/xyz/song.mp3?vuutv=tok";