fix(client/apple): the mic was never in the graph, so nothing ever pulled it

ROOT CAUSE, from the reporter's device log:

  16:25:49.093  mic capture: 48000 Hz, 1 ch          <- tap installed, format fine
  16:25:49.235  audio engines joined - voice processing active
  ... 13 s of session, no errors, and the 10 s silence verdict NEVER fires

The engine started clean and the tap was installed against a valid
format - so neither the format timing nor the encoder was the fault. The
tripwire fires after ten seconds of CAPTURED frames and never fired
across a 13-second session: the tap received nothing at all.

Because the capture side must be pulled, and only the render graph pulls
anything. On the combined engine the input node carried a tap and no
connection, so it was not in the graph and nobody drove it: the IO unit
came up (the recording indicator lit for a beat, then went out as the
input went idle) and not one buffer ever reached the tap. No error, no
failed start - a session that quietly sent no microphone.

The input now runs through a silent sink into the main mixer, which is
what Apple's own voice-processing sample does. outputVolume = 0 because
the mic must reach the graph and never the speaker. The split path never
needed this - a capture-only engine has the input node AS its graph - so
this broke exactly when the combined topology became the default.

Verified: swift build (macOS), swift build --triple arm64-apple-ios17.0,
swift test 208 passed. Awaiting the reporter's on-device confirmation.

clients/apple/Sources/PunktfunkKit/Audio/SessionAudio.swift

@@ -483,6 +483,25 @@ public final class SessionAudio {
         }
         engine.attach(source)
         engine.connect(source, to: engine.mainMixerNode, format: format)
+
+        // The capture side must be PULLED, and only the render graph pulls anything. An input
+        // node carrying nothing but a tap is not part of that graph, so on the combined engine
+        // nobody drove it: the IO unit came up (the recording indicator lit for a beat, then went
+        // out as the input went idle) and NOT ONE BUFFER ever reached the tap — no error, no
+        // failed start, just a session that quietly sent no microphone at all. Routing the input
+        // through a silent sink puts it in the graph, which is what Apple's own voice-processing
+        // sample does. The split path never needed it: a capture-only engine has the input node
+        // AS its graph, so it is pulled by definition — which is why this only broke when the
+        // combined topology became the default.
+        //
+        // `outputVolume = 0` on the sink: the mic has to reach the graph, never the speaker. At
+        // any audible volume this is a microphone wired straight to the earpiece.
+        let micSink = AVAudioMixerNode()
+        engine.attach(micSink)
+        micSink.outputVolume = 0
+        engine.connect(engine.inputNode, to: micSink, format: nil)
+        engine.connect(micSink, to: engine.mainMixerNode, format: nil)
+
         // BEFORE the tap reads a format. Enabling voice processing swaps the engine's IO unit
         // for the VPIO one and renegotiates its formats, and until the engine is prepared the
         // input node can still report the pre-swap state — 0 Hz / 0 channels included, which