feat(latency): wall-clock skew handshake for cross-machine latency measurement

ClockProbe/ClockEcho on the QUIC control stream — 8 NTP-style rounds right after
Start; the min-RTT sample gives the host-client clock offset (clock_offset_ns
estimator in punktfunk-core). The client adds the offset to its receive instant
before differencing against the AU pts_ns, so the capture->reassembled latency
percentiles are valid across machines (skew_corrected=true), not just same-host.
Back-compat: an old host that doesn't answer the probe times out and the client
falls back to a shared-clock assumption (skew_corrected=false).

Host adds one ClockProbe dispatch arm in the control task; the client runs
clock_sync after Start, before the --remode/--speed-test tasks take the stream.

Validated cross-LAN (GNOME box -> dev box): offset ~ -1.57 ms (reproducible),
rtt ~140 us, p50 1.30 ms skew-corrected capture->reassembled — the offset is
exactly the systematic error the handshake removes. Unit tests for the message
codecs and the min-RTT offset estimator.

Roadmap §12: skew handshake done; remaining for true glass-to-glass is the Apple
client present-stamp (decode->present) plus the host render->capture term.

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

include/punktfunk_core.h

@@ -166,6 +166,16 @@
 #define MSG_PROBE_RESULT 33
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Type byte of [`ClockProbe`].
+#define MSG_CLOCK_PROBE 48
+#endif
+
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Type byte of [`ClockEcho`].
+#define MSG_CLOCK_ECHO 49
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Type byte of [`PairRequest`].
 #define MSG_PAIR_REQUEST 16