feat(apple): capture->client latency HUD (skew-corrected) via the connect offset

The Apple client now consumes the connector's clock offset. PunktfunkConnection
reads punktfunk_connection_clock_offset_ns into clockOffsetNs at connect; a new
LatencyMeter (PunktfunkKit, NSLock + percentiles, mirrors FrameMeter) records each
AU's capture->client-receipt latency = now(CLOCK_REALTIME) + offset - pts_ns, and
SessionModel drains p50/p95 into the macOS HUD ("capture->client N/N ms p50/p95",
"(same-host)" when the host didn't answer the skew handshake). Wired at the
existing onFrame hook in ContentView — additive, no change to the decode/present
path. Unit test for the meter (percentiles, skew flag, absurd-value guard).

This is the first cross-machine latency the real Apple client reports. SCOPE:
stage-1 AVSampleBufferDisplayLayer decodes+presents compressed samples internally
with no per-frame callback, so this excludes decode+present; true decode->present
needs the stage-2 presenter (VTDecompressionSession + CAMetalLayer). Rebuild
PunktfunkCore.xcframework (for the new C getter) before swift build/test on a Mac.

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

clients/apple/Sources/PunktfunkClient/ContentView.swift

@@ -478,7 +478,10 @@ struct ContentView: View {
                     onCaptureChange: { [weak model] captured in
                         model?.mouseCaptured = captured
                     },
-                    onFrame: { [meter = model.meter] au in meter.note(byteCount: au.data.count) },
+                    onFrame: { [meter = model.meter, latency = model.latency, offset = conn.clockOffsetNs] au in
+                        meter.note(byteCount: au.data.count)
+                        latency.record(ptsNs: au.ptsNs, offsetNs: offset)
+                    },
                     onSessionEnd: { [weak model] in
                         Task { @MainActor in model?.sessionEnded() }
                     }
@@ -499,6 +502,14 @@ struct ContentView: View {
                 Text("\(conn.width)×\(conn.height)@\(conn.refreshHz)  \(model.fps) fps  \(model.mbps, specifier: "%.1f") Mb/s")
                     .font(.system(.caption, design: .monospaced))
             }
+            if model.latencyValid {
+                // Capture→client-receipt (skew-corrected); excludes the layer's decode+present —
+                // see LatencyMeter. "(same-host)" when the host didn't answer the skew handshake.
+                Text("capture→client \(model.latencyP50Ms, specifier: "%.1f")/\(model.latencyP95Ms, specifier: "%.1f") ms p50/p95"
+                    + (model.latencySkewCorrected ? "" : " (same-host)"))
+                    .font(.system(.caption2, design: .monospaced))
+                    .foregroundStyle(.secondary)
+            }
             // While captured the cursor is hidden+frozen, so the button is keyboard-only
             // (⌘⎋ or Cmd+Tab release the cursor; released, it's clickable again).
             #if os(macOS)