feat(sound): snapshot getter + volume-update event on SoundManager

Extends the snapshot pattern to the three audio volume levels (system /
furni / trax) so volume-slider widgets on the React client can subscribe
to a single source of truth via useSyncExternalStore.

API additions on ISoundManager:
- systemVolume / furniVolume getters (parity with the existing
  traxVolume getter)
- getVolumesSnapshot(): Readonly<ISoundVolumesSnapshot> with the same
  lazy-frozen + invalidation-on-change semantics as the user/session
  snapshots
- new ISoundVolumesSnapshot { system, furni, trax } interface

New event: NitroEventType.SOUND_VOLUMES_UPDATED. Dispatched only when
the incoming NitroSettingsEvent.SETTINGS_UPDATED actually changes one
of the three volumes (a no-op refresh stays quiet).

While in there, fixed a real bug: the previous implementation cached
`volumeFurniUpdated` / `volumeTraxUpdated` BEFORE writing the new
values, but read `castedEvent.volumeFurni` / `castedEvent.volumeTrax`
in their pre-division form — comparing percent (e.g. 75) against the
already-divided stored value (e.g. 0.75) — so the change check almost
always reported "updated" for a real settings push and never reported
"updated" if the percent matched the stored fraction by coincidence
(only 0/100 are stable). Updated check is now consistent (compare
fraction vs fraction) and also tracks systemVolume changes for the
new snapshot invalidation.

packages/events/src/NitroEventType.ts

@@ -22,4 +22,5 @@ export class NitroEventType
     public static readonly IGNORED_USERS_UPDATED = 'IGNORED_USERS_UPDATED';
     public static readonly GROUP_BADGES_UPDATED = 'GROUP_BADGES_UPDATED';
     public static readonly ROOM_USER_LIST_UPDATED = 'ROOM_USER_LIST_UPDATED';
+    public static readonly SOUND_VOLUMES_UPDATED = 'SOUND_VOLUMES_UPDATED';
 }