feat(core,host): adaptive bitrate — mid-stream encoder re-targeting when set to Automatic

New SetBitrate (0x05) / BitrateChanged (0x06) control messages: the host clamps like the
Hello request, acks the resolved rate, and rebuilds the ENCODER ONLY in place (same mode,
first new-rate frame is an IDR — the proven mode-switch resync, minus the pipeline churn).
The client side is an AIMD controller (core abr.rs) in the data-plane pump, armed only when
the user's bitrate is Automatic (Hello bitrate_kbps == 0): ×0.7 after two bad 750 ms windows
(FEC-unrecoverable frames, ≥2% loss, one-way-delay rise above its rolling baseline — the
pre-loss bufferbloat signal off the clock-skew handshake — or a jump-to-live flush), ~+6%
after ~10 s clean, ceiling = the session's starting rate, 3 s cooldown, self-disables against
a host that never acks (older build). Division of labour: adaptive FEC keeps answering fast
random loss; bitrate now answers persistent congestion, closing the FEC death-spiral gap.
The web-console sample reports the live rate. Also: join_host_port() brackets bare IPv6
literals before SocketAddr parsing (parse-side IPv6 groundwork, pairs with the next commit).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

include/punktfunk_core.h

@@ -356,6 +356,16 @@
 #define MSG_LOSS_REPORT 4
 #endif
 
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Type byte of [`SetBitrate`].
+#define MSG_SET_BITRATE 5
+#endif
+
+#if defined(PUNKTFUNK_FEATURE_QUIC)
+// Type byte of [`BitrateChanged`].
+#define MSG_BITRATE_CHANGED 6
+#endif
+
 #if defined(PUNKTFUNK_FEATURE_QUIC)
 // Type byte of [`ProbeRequest`].
 #define MSG_PROBE_REQUEST 32