feat(audio): per-stream mix controls in Windows client + vc_get_remote_stream getter

PerUserTuningDialog previously broadcast one gain/mute/NR set to *all* of a
user's streams, even though the core mixer (AudioEngine::RemoteStream) and
the C ABI (vc_set_remote_stream) were already per-stream. The UI had no
per-mix controls anywhere.

Reworks the dialog to enumerate ListUserStreams on open and render one row
per stream (kind + label + Gain + Mute + NR), each wiring only to its own
stream_id. Adds a read-back ABI counterpart, vc_get_remote_stream, so the
dialog opens at the listener's actual current per-stream settings (defaults
1.0/unmuted/NR-off) rather than always 100%. Additive ABI change only; no
existing symbols touched.

Tests: test_m3_multistream extended with getter round-trip assertions; new
C# smoke test exercises the full P/Invoke marshaling path with two clients.
Docs: voice.md §10 notes the getter. NR checkbox keeps its honest
'passthrough' label (NS DSP still unbuilt per §8).

docs/voice.md

@@ -214,6 +214,11 @@ it is a local UI action with **no protocol message** and no effect on other list
 each receive stream is decoded independently before the mixer (voice.md §1), per-user receive
 NS is a clean drop-in on that per-stream stage.
 
+All three receive-side controls (gain, mute, NR) are queryable via `vc_get_remote_stream` —
+the counterpart to `vc_set_remote_stream` — so a UI can reopen its per-stream mix controls at
+the listener's actual current settings (defaults: gain 1.0, unmuted, NR off). Like the setter,
+it carries no protocol traffic.
+
 ## 11. Input activation — VAD and PTT (client-configurable)
 
 Whether the mic transmits is decided locally by the **input gate**, and the client supports