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>

docs-site/content/docs/status.md

@@ -29,6 +29,11 @@ All three appliances advertise over mDNS (`_punktfunk._udp`) and require PIN pai
 ## Progress log
 
 ### 2026-06-12
+- **Apple client latency HUD** — `PunktfunkConnection.clockOffsetNs` (from the C-ABI getter) +
+  `LatencyMeter` surface a skew-corrected **capture→client-receipt** p50/p95 in the macOS HUD: the
+  first cross-machine latency the real Apple client reports. (Stage-1 `AVSampleBufferDisplayLayer`
+  has no present callback, so decode→present is excluded — that needs the stage-2 presenter.)
+  Needs an `xcframework` rebuild + `swift test` on the Mac to validate.
 - **Skew handshake in the connector + C ABI** — `quic::clock_sync` is now a shared core helper used
   by both the reference client and `NativeClient`; the connector runs it at connect and exposes the
   host clock offset over the C ABI (`punktfunk_connection_clock_offset_ns`). This is the substrate