[TC-1] Voice mutation API has zero tests at any tier #414

From #413.

Sev: high. Tier: client. Obvious?: yes.

crates/client/src/mutations.rs voice mutators ship with no test:

• join_voice (line 646)
• leave_voice (line 669)
• toggle_mute (line 694)
• toggle_deafen (line 703)
• voice_peer_joined (line 796)
• voice_peer_left (line 811)

Evidence:

$ rg "voice|toggle_mute|toggle_deafen" -l crates/client/src/tests/
(empty)
$ rg "voice|toggle_mute|toggle_deafen" -l e2e/
(empty)

Voice presence feeds UI state. Mute/deafen/channel-switch regressions silent.

Fix: crates/client/src/tests/voice.rs per CLAUDE.md tier-2 (client). One happy-path test per mutator using test_client().