feat(connector): expose host clock offset over the C ABI for glass-to-glass

Factor the client-side skew handshake into a shared core helper (quic::clock_sync
-> ClockSkew) so both the reference client and the embeddable connector use one
implementation. NativeClient now runs the handshake at connect (right after Start,
before the control task takes the stream) and stores the host-client offset; it's
read over the C ABI via punktfunk_connection_clock_offset_ns (i64 ns, host minus
client; 0 = no correction / old host).

This is the substrate the Apple client needs for the decode->present (glass-to-
glass) term: stamp present time, add the offset to express it in the host's
capture clock, subtract the AU pts_ns. client-rs drops its local clock_sync copy
and uses the shared helper (behavior unchanged; validated locally).

Regenerates include/punktfunk_core.h. Roadmap section 12 + status updated.

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

include/punktfunk_core.h

@@ -797,6 +797,21 @@ PunktfunkStatus punktfunk_connection_gamepad(const PunktfunkConnection *c, uint3
 PunktfunkStatus punktfunk_connection_bitrate(const PunktfunkConnection *c, uint32_t *bitrate_kbps);
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// The host↔client wall-clock offset (nanoseconds, **host minus client**) measured by the
+// connect-time skew handshake. Add it to a local receive/present timestamp (same realtime clock,
+// `CLOCK_REALTIME` / `gettimeofday`-epoch nanoseconds) to express that instant in the host's
+// capture clock — the clock the per-access-unit `pts_ns` is stamped in — so glass-to-glass latency
+// (e.g. present-time minus `pts_ns`) is valid across machines. `0` = no correction: either an older
+// host that didn't answer the handshake, or genuinely synchronized clocks. Safe any time after
+// connect.
+//
+// # Safety
+// `c` is a valid connection handle; `offset_ns` is writable (NULL is skipped).
+PunktfunkStatus punktfunk_connection_clock_offset_ns(const PunktfunkConnection *c,
+                                                     int64_t *offset_ns);
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Ask the host to switch the live session to `width`x`height`@`refresh_hz` without
 // reconnecting (window resized, refresh changed). Non-blocking enqueue: on acceptance the