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>

CLAUDE.md

@@ -50,7 +50,11 @@ Low-latency desktop/game streaming stack, Linux-first, with a shared Rust protoc
   **Mid-stream mode renegotiation**: `Reconfigure` on the still-open control stream — the
   host rebuilds output+encoder at the new mode in ~90 ms while the data plane runs on
   (validated live: one .h265 with 720p and 1080p segments). Measured on-box at 720p120: 1680/1680 frames, **p50 0.83 ms**
-  capture→…→reassembled; audio measured live (~200 pkts/s). `punktfunk-client-rs` is the
+  capture→…→reassembled; audio measured live (~200 pkts/s). A **wall-clock skew handshake**
+  (`ClockProbe`/`ClockEcho`, 8 NTP rounds after `Start`, `clock_offset_ns`) aligns the client to the
+  host clock, so that latency is now valid **cross-machine** (`skew_corrected=true`) — measured GNOME
+  box → dev box over the LAN: **p50 1.30 ms** (the −1.57 ms inter-box clock offset removed).
+  `punktfunk-client-rs` is the
   working reference client (`--pin`, datagram counters, `--input-test` incl. gamepad).
   The embeddable connector (`NativeClient`) exposes it all over the C ABI: `punktfunk_connect`
   (pin/TOFU) + `next_au`/`next_audio`/`next_rumble`/`next_hidout`/`send_input`/