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/roadmap.md

@@ -306,11 +306,14 @@ buffer; `sendmmsg`/`recvmmsg` batching; the capture-timestamp anchor placement.
   **across machines** (reported `skew_corrected=true`), not just same-host. Back-compat: an old host
   that doesn't answer times out → `skew_corrected=false` (shared-clock assumption, as before).
   Validated cross-LAN (GNOME box → dev box): offset ≈ −1.57 ms (reproducible), rtt ~140 µs, **p50
-  1.30 ms** skew-corrected capture→reassembled. **Remaining for true glass-to-glass**: the **client
-  present-stamp** (decode→present term) — only the Apple client presents today, so it needs the
-  connector to expose the offset + an Apple present-time probe; and the **render→capture** term
-  (PipeWire buffer presentation timestamp vs our capture stamp). `tools/latency-probe` is still the
-  cross-machine orchestrator.
+  1.30 ms** skew-corrected capture→reassembled. The skew handshake is now a shared core helper
+  (`quic::clock_sync` → `ClockSkew`) used by both the reference client and the **embeddable
+  connector** — `NativeClient` runs it at connect and exposes the offset over the C ABI
+  (`punktfunk_connection_clock_offset_ns`), so the Apple client can convert a present instant to the
+  host clock. **Remaining for true glass-to-glass**: (1) the **Apple client present-stamp**
+  (decode→present) — Swift: stamp `AVSampleBufferDisplayLayer`/presenter time, add the C-ABI offset,
+  subtract the AU `pts_ns`; (2) the host **render→capture** term (PipeWire buffer presentation
+  timestamp vs our capture stamp). `tools/latency-probe` is still the cross-machine orchestrator.
 - **Bigger bets (ordered, deferred — need real-NIC/GPU/Mac validation):**
   1. **CUDA stream+event** to drop one of two redundant `cuCtxSynchronize` in `submit_cuda` (keep the
      copy) — ~0.1–0.4 ms@720p, ~1 ms@5K; only if per-stage timing proves the sync is on the path.