server: upstream trains, observed TTL/DSCP/ECN, rate limits, action ids

Types 0x03/0x04/0x05 land with a bounded columnar train buffer (head kept,
truncation declared) and grant-free multi-part reports - a report row is
smaller than the packet it answers, so $3.4 holds without a grant. The
read loop now collects TTL/TOS cmsgs on Linux, replacing the 0xFF stubs in
the observation block with what the kernel saw; downtrain gained a dscp
parameter, so DSCP survival is measurable in both directions.

Rate limiting ($2.5) exists now: per-credential AND per-source buckets,
429 on the control plane, silent drop on the data plane after the HMAC
gate and before the replay window. UDP ceilings default above the largest
legitimate run - a limit that clips a real measurement produces a
confidently wrong number.

Every granted packet carries its action_id at payload[8:16]; overlapping
actions were unattributable before. Canary DNS logs now honor the stated
24h privacy default. /admin/enroll-tokens answers the spec's JSON shape.
protocol_version 1.0.1 (additive).

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

server/internal/dataplane/cmsg_linux.go

@@ -0,0 +1,70 @@
+// SPDX-FileCopyrightText: 2026 Echolot contributors
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build linux
+
+package dataplane
+
+import (
+	"encoding/binary"
+	"net"
+	"syscall"
+)
+
+// Per-packet TTL and TOS/traffic-class arrive as control messages, and only if asked for at
+// socket setup. These fill the spec §3.3 observation-block fields that were shipped as the 0xFF
+// sentinel until now — received TTL is path-length evidence, the TOS byte is DSCP/ECN survival.
+
+// enableRecvMeta asks the kernel to attach the cmsgs to every received datagram. Both the v4 and
+// the v6 option sets are attempted on every socket: a dual-stack socket delivers v4-mapped
+// traffic through the v6 fd, and the kernel refuses whichever set does not apply. Errors are
+// dropped on purpose — a socket that cannot deliver metadata still serves probes, and the
+// sentinel already says "not observed" for it.
+func enableRecvMeta(conn *net.UDPConn) {
+	raw, err := conn.SyscallConn()
+	if err != nil {
+		return
+	}
+	_ = raw.Control(func(fd uintptr) {
+		_ = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_RECVTTL, 1)
+		_ = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IP, syscall.IP_RECVTOS, 1)
+		_ = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, syscall.IPV6_RECVHOPLIMIT, 1)
+		_ = syscall.SetsockoptInt(int(fd), syscall.IPPROTO_IPV6, syscall.IPV6_RECVTCLASS, 1)
+	})
+}
+
+// parseMeta extracts TTL and the TOS byte from one datagram's control messages. Anything absent
+// or unparseable keeps the sentinel — reported as unobserved, never guessed.
+func parseMeta(oob []byte) pktMeta {
+	m := pktMeta{TTL: metaUnavailable, TOS: metaUnavailable}
+	if len(oob) == 0 {
+		return m
+	}
+	cmsgs, err := syscall.ParseSocketControlMessage(oob)
+	if err != nil {
+		return m
+	}
+	for _, c := range cmsgs {
+		switch {
+		case c.Header.Level == syscall.IPPROTO_IP && c.Header.Type == syscall.IP_TTL,
+			c.Header.Level == syscall.IPPROTO_IPV6 && c.Header.Type == syscall.IPV6_HOPLIMIT:
+			m.TTL = cmsgValue(c.Data)
+		case c.Header.Level == syscall.IPPROTO_IP && c.Header.Type == syscall.IP_TOS,
+			c.Header.Level == syscall.IPPROTO_IPV6 && c.Header.Type == syscall.IPV6_TCLASS:
+			m.TOS = cmsgValue(c.Data)
+		}
+	}
+	return m
+}
+
+// cmsgValue reads a cmsg the kernel encodes either as a native-endian int (IP_TTL,
+// IPV6_HOPLIMIT, IPV6_TCLASS) or as a single byte (IP_TOS). Both fit a byte by definition.
+func cmsgValue(data []byte) uint8 {
+	switch {
+	case len(data) >= 4:
+		return uint8(binary.NativeEndian.Uint32(data))
+	case len(data) >= 1:
+		return data[0]
+	}
+	return metaUnavailable
+}