feat: support relative volume adjustment (+N/-N)

!volume +10 increases by 10, !volume -5 decreases by 5.
Out-of-range results (below 0 or above 100) are rejected.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_music.py

@@ -319,8 +319,41 @@ class TestVolumeCommand:
         asyncio.run(_mod.cmd_volume(bot, msg))
         assert any("0-100" in r for r in bot.replied)
 
-    def test_volume_negative(self):
+    def test_volume_negative_absolute(self):
+        """Bare negative that underflows clamps at 0-100 error."""
         bot = _FakeBot()
+        _mod._ps(bot)["volume"] = 5
+        msg = _Msg(text="!volume -10")
+        asyncio.run(_mod.cmd_volume(bot, msg))
+        assert any("0-100" in r for r in bot.replied)
+
+    def test_volume_relative_up(self):
+        bot = _FakeBot()
+        msg = _Msg(text="!volume +15")
+        asyncio.run(_mod.cmd_volume(bot, msg))
+        ps = _mod._ps(bot)
+        assert ps["volume"] == 65
+        assert any("65%" in r for r in bot.replied)
+
+    def test_volume_relative_down(self):
+        bot = _FakeBot()
+        _mod._ps(bot)["volume"] = 80
+        msg = _Msg(text="!volume -20")
+        asyncio.run(_mod.cmd_volume(bot, msg))
+        ps = _mod._ps(bot)
+        assert ps["volume"] == 60
+        assert any("60%" in r for r in bot.replied)
+
+    def test_volume_relative_clamp_over(self):
+        bot = _FakeBot()
+        _mod._ps(bot)["volume"] = 95
+        msg = _Msg(text="!volume +10")
+        asyncio.run(_mod.cmd_volume(bot, msg))
+        assert any("0-100" in r for r in bot.replied)
+
+    def test_volume_relative_clamp_under(self):
+        bot = _FakeBot()
+        _mod._ps(bot)["volume"] = 5
         msg = _Msg(text="!volume -10")
         asyncio.run(_mod.cmd_volume(bot, msg))
         assert any("0-100" in r for r in bot.replied)