fix(apple-client/audio): capture the right channel of a multi-channel mic + diagnostics

The mic uplink handed the host pure digital silence on a multi-channel
interface: AVAudioConverter's N→stereo downmix takes channels 0/1, but a
pro interface puts the mic on ONE higher discrete channel. Fold the input
to a mono bus ourselves instead — pick the mic's channel (or sum all) and
resample that to the encoder's 48 kHz stereo, so the silent 0/1 downmix
never happens.

- New "Microphone channel" setting (macOS): Auto (sum every channel — a
  lone hot mic passes at full level) or pin 1-based channel N. Picker
  appears only for multi-channel devices, driven by the device's input
  channel count.
- Diagnostics that make this class of failure self-naming next session:
  log the actual live capture device + format + fold mode, warn on a
  silent UID fallback, and a one-shot silence tripwire on the EXTRACTED
  signal (WARN on 10 s of zeros, else peak dBFS).
- foldToMono extracted as a pure, unit-tested helper (pin / sum-clamp x
  interleaved / deinterleaved / mono / out-of-range).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/apple/Tests/PunktfunkKitTests/AudioChannelFoldTests.swift

@@ -0,0 +1,93 @@
+// Multi-channel input → mono fold (SessionAudio.foldToMono): the fix for a mic on one channel of
+// a multi-channel interface. AVAudioConverter's default N→stereo downmix grabs channels 0/1 — dead
+// silence when the mic sits higher up — so we fold ourselves. This pins the fiddly bits (the
+// interleaved stride, channel pinning, the sum-clamp) against regressions without needing hardware.
+
+#if !os(tvOS)
+import XCTest
+
+@testable import PunktfunkKit
+
+final class AudioChannelFoldTests: XCTestCase {
+    /// Drive `foldToMono` over channel data expressed as `[[Float]]`, mirroring the two
+    /// `floatChannelData` layouts:
+    ///   - deinterleaved: each inner array is one channel (all `frames` long).
+    ///   - interleaved: a single inner array already interleaved (c0f0, c1f0, …), with the real
+    ///     channel count passed separately.
+    private func fold(
+        _ planes: [[Float]], frames: Int, channels: Int, interleaved: Bool, pinned: Int?
+    ) -> [Float] {
+        // One C buffer per plane + a table of pointers to them — the shape of floatChannelData.
+        let buffers: [UnsafeMutablePointer<Float>] = planes.map { plane in
+            let p = UnsafeMutablePointer<Float>.allocate(capacity: plane.count)
+            for i in 0..<plane.count { p[i] = plane[i] }
+            return p
+        }
+        let table = UnsafeMutablePointer<UnsafeMutablePointer<Float>>.allocate(
+            capacity: buffers.count)
+        for (i, b) in buffers.enumerated() { table[i] = b }
+        let out = UnsafeMutablePointer<Float>.allocate(capacity: frames)
+        defer {
+            buffers.forEach { $0.deallocate() }
+            table.deallocate()
+            out.deallocate()
+        }
+        SessionAudio.foldToMono(
+            input: table, frames: frames, channels: channels,
+            interleaved: interleaved, pinned: pinned, out: out)
+        return (0..<frames).map { out[$0] }
+    }
+
+    // A pinned channel is copied verbatim — the exact fix: mic on a HIGH channel, not 0/1.
+    func testPinsHigherChannelDeinterleaved() {
+        let result = fold(
+            [[0, 0, 0], [0, 0, 0], [0.1, 0.2, 0.3], [0, 0, 0]],
+            frames: 3, channels: 4, interleaved: false, pinned: 2)
+        XCTAssertEqual(result, [0.1, 0.2, 0.3])
+    }
+
+    // Same signal, interleaved layout: [c0f0,c1f0,c2f0,c3f0, c0f1,…]. Guards the `i*ch + c` stride.
+    func testPinsHigherChannelInterleaved() {
+        let interleaved: [Float] = [
+            0, 0, 0.1, 0,
+            0, 0, 0.2, 0,
+            0, 0, 0.3, 0,
+        ]
+        let result = fold([interleaved], frames: 3, channels: 4, interleaved: true, pinned: 2)
+        XCTAssertEqual(result, [0.1, 0.2, 0.3])
+    }
+
+    // Auto (pinned: nil): a lone hot channel amid silence passes at FULL level, never attenuated.
+    func testAutoSumsAllChannelsSoALoneMicSurvives() {
+        let result = fold(
+            [[0, 0], [0.4, -0.4], [0, 0]],
+            frames: 2, channels: 3, interleaved: false, pinned: nil)
+        XCTAssertEqual(result, [0.4, -0.4])
+    }
+
+    // Two simultaneously-hot channels sum past the unit range → clamped, never wraps/overflows.
+    func testAutoSumClampsToUnitRange() {
+        let result = fold(
+            [[0.8, -0.8], [0.9, -0.9]],
+            frames: 2, channels: 2, interleaved: false, pinned: nil)
+        XCTAssertEqual(result, [1.0, -1.0])
+    }
+
+    // A plain mono device is passed through untouched (no clamp, no attenuation).
+    func testMonoIsIdentity() {
+        let result = fold(
+            [[0.25, -0.5, 0.75]], frames: 3, channels: 1, interleaved: false, pinned: nil)
+        XCTAssertEqual(result, [0.25, -0.5, 0.75])
+    }
+
+    // Belt-and-suspenders: an out-of-range pin (the tap already guards, but the setting is
+    // persisted) is ignored by foldToMono's own `ch < channels` guard, which sums instead of
+    // reading past the buffer.
+    func testOutOfRangePinFallsBackToSum() {
+        let result = fold(
+            [[0, 0], [0.3, 0.3]],
+            frames: 2, channels: 2, interleaved: false, pinned: 2)
+        XCTAssertEqual(result, [0.3, 0.3])
+    }
+}
+#endif