feat(client/speedtest): request the host's full 3 Gbps probe ceiling

The Apple speed test asked for only 400 Mbps, capping the measured throughput
there and hiding the link's real headroom. Request the host's full
MAX_PROBE_KBPS (3 Gbps) instead, and raise the recommended-bitrate clamp from
500 Mbps to the host's 2 Gbps session ceiling so a fast measurement yields a
usable recommendation.

Also fix the stale caps left when the host clamps were raised (b8a33e2): the
resolved-bitrate range and the probe doc comments (abi.rs, client.rs,
regenerated header), plus the section 9 roadmap copy, now read 3 Gbps probe /
2 Gbps session.

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

include/punktfunk_core.h

@@ -830,7 +830,7 @@ PunktfunkStatus punktfunk_connection_request_mode(const PunktfunkConnection *c,
 
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Start a bandwidth speed test: ask the host to burst filler over the data plane at
-// `target_kbps` of goodput for `duration_ms` (each clamped host-side to ≤ 1 Gbps / ≤ 5 s),
+// `target_kbps` of goodput for `duration_ms` (each clamped host-side to ≤ 3 Gbps / ≤ 5 s),
 // *briefly pausing video*. Non-blocking — poll [`punktfunk_connection_probe_result`] until its
 // `done` field is 1. Starting a probe resets any prior measurement.
 //