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/PunktfunkKit/PunktfunkConnection.swift

@@ -195,6 +195,13 @@ public final class PunktfunkConnection {
     /// DualSense feedback.
     public private(set) var resolvedGamepad: GamepadType = .auto
 
+    /// Host clock minus client clock (nanoseconds), from the connect-time wall-clock skew handshake
+    /// (`punktfunk_connection_clock_offset_ns`). Add it to a local `CLOCK_REALTIME` instant to
+    /// express that instant in the host's capture clock — the clock each `AccessUnit.ptsNs` is
+    /// stamped in — so a glass-to-glass latency (present/enqueue time minus `ptsNs`) is valid across
+    /// machines. `0` = no correction (an older host that didn't answer, or synchronized clocks).
+    public private(set) var clockOffsetNs: Int64 = 0
+
     /// Connect and start a session at the requested mode (the host creates a native virtual
     /// output at exactly this size/refresh). Blocks up to `timeoutMs`.
     ///
@@ -251,6 +258,9 @@ public final class PunktfunkConnection {
         var gp: UInt32 = 0
         _ = punktfunk_connection_gamepad(handle, &gp)
         resolvedGamepad = GamepadType(rawValue: gp) ?? .auto
+        var offset: Int64 = 0
+        _ = punktfunk_connection_clock_offset_ns(handle, &offset)
+        clockOffsetNs = offset
     }
 
     /// Ask the host to switch the live session to a new mode (window resized) — no