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>

docs-site/content/docs/status.md

@@ -29,6 +29,10 @@ All three appliances advertise over mDNS (`_punktfunk._udp`) and require PIN pai
 ## Progress log
 
 ### 2026-06-12
+- **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
+  the Apple client needs for the decode→present (glass-to-glass) term.
 - **Wall-clock skew handshake** (`ClockProbe`/`ClockEcho`, 8 NTP rounds after `Start`) — makes the
   client's capture→reassembled latency valid **cross-machine**. Validated GNOME box → dev box:
   offset −1.57 ms removed, **p50 1.30 ms** skew-corrected. (`05bc9ab`)