Move settings to Settings.json and add master volume

Replace the former per-value PlayerPrefs storage with a shared human-readable
Settings.json store (GameSettings). MouseSensitivity keeps its public .Multiplier
API and only swaps its backing to the JSON store, so the camera controllers stay
untouched. Add a MasterVolume facade that drives the AudioMixer's exposed
"MasterVolume" parameter (loaded from Resources, linear<->dB mapping), applied at
startup and on change, plus a MasterVolumeSlider binder. The default is the level
currently authored on the mixer instead of an arbitrary constant.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Resources/AudioMixer.mixer

@@ -0,0 +1,71 @@
+%YAML 1.1
+%TAG !u! tag:unity3d.com,2011:
+--- !u!241 &24100000
+AudioMixerController:
+  m_ObjectHideFlags: 0
+  m_CorrespondingSourceObject: {fileID: 0}
+  m_PrefabInstance: {fileID: 0}
+  m_PrefabAsset: {fileID: 0}
+  m_Name: AudioMixer
+  m_OutputGroup: {fileID: 0}
+  m_MasterGroup: {fileID: 24300002}
+  m_Snapshots:
+  - {fileID: 24500006}
+  m_StartSnapshot: {fileID: 24500006}
+  m_SuspendThreshold: -80
+  m_EnableSuspend: 1
+  m_UpdateMode: 0
+  m_ExposedParameters:
+  - guid: e1c3ba92ff34ded45b9fb0e7e9d9ff5f
+    name: MasterVolume
+  m_AudioMixerGroupViews:
+  - guids:
+    - baf4df5f8e456a244865f2490b8be297
+    name: View
+  m_CurrentViewIndex: 0
+  m_TargetSnapshot: {fileID: 24500006}
+--- !u!243 &24300002
+AudioMixerGroupController:
+  m_ObjectHideFlags: 0
+  m_CorrespondingSourceObject: {fileID: 0}
+  m_PrefabInstance: {fileID: 0}
+  m_PrefabAsset: {fileID: 0}
+  m_Name: Master
+  m_AudioMixer: {fileID: 24100000}
+  m_GroupID: baf4df5f8e456a244865f2490b8be297
+  m_Children: []
+  m_Volume: e1c3ba92ff34ded45b9fb0e7e9d9ff5f
+  m_Pitch: 55a908e3384754b40ab5380d73216150
+  m_Send: 00000000000000000000000000000000
+  m_Effects:
+  - {fileID: 24400004}
+  m_UserColorIndex: 0
+  m_Mute: 0
+  m_Solo: 0
+  m_BypassEffects: 0
+--- !u!244 &24400004
+AudioMixerEffectController:
+  m_ObjectHideFlags: 3
+  m_CorrespondingSourceObject: {fileID: 0}
+  m_PrefabInstance: {fileID: 0}
+  m_PrefabAsset: {fileID: 0}
+  m_Name: 
+  m_EffectID: e9eec667a6b554a43b51e2771497e881
+  m_EffectName: Attenuation
+  m_MixLevel: 83ef66f0d6268ba47998027f81758c3b
+  m_Parameters: []
+  m_SendTarget: {fileID: 0}
+  m_EnableWetMix: 0
+  m_Bypass: 0
+--- !u!245 &24500006
+AudioMixerSnapshotController:
+  m_ObjectHideFlags: 0
+  m_CorrespondingSourceObject: {fileID: 0}
+  m_PrefabInstance: {fileID: 0}
+  m_PrefabAsset: {fileID: 0}
+  m_Name: Snapshot
+  m_AudioMixer: {fileID: 24100000}
+  m_SnapshotID: 2cd06a6f291b6c8468486aebc1437c6b
+  m_FloatValues:
+    e1c3ba92ff34ded45b9fb0e7e9d9ff5f: -6
+  m_TransitionOverrides: {}

Assets/Resources/AudioMixer.mixer.meta

@@ -0,0 +1,8 @@
+fileFormatVersion: 2
+guid: 4c2f5be6ec68f3e4aa5ef0471d7f2e22
+NativeFormatImporter:
+  externalObjects: {}
+  mainObjectFileID: 24100000
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: