server: Go skeleton — control plane, UDP data plane, Docker + systemd modes

Pure stdlib. Implements the spec's core: enrollment (single-use tokens),
profile (SPKI pin, only real capabilities advertised), sessions with the
§2.4 HKDF-SHA256 key schedule; UDP data plane with the 32-byte ELT1
header, 4-byte HMAC gate, 1024-wide anti-replay window, ECHO_RESP with
observation block, TIMESYNC, and the §3.4 anti-amplification cap. Wire
format has tests (roundtrip + silent-drop cases); enroll→profile→session
smoke-tested live.

Modes: container (autodetect /.dockerenv|/run/.containerenv|cgroup, or
--docker/ECHOLOT_DOCKER=1; config via ECHOLOT_* env; distroless image;
network_mode host required — Docker NAT would falsify observed sources)
and native (--install-systemd/--uninstall-systemd with a hardened unit,
opt-in --self-update from Gitea releases; refused in containers).

CI: tests on any server/ push; server-v* tags build+push the image to the
Gitea registry and attach linux amd64/arm64 binaries + SHA256SUMS to a
release — the artifact self-update consumes.

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

server/Dockerfile

@@ -0,0 +1,25 @@
+# SPDX-FileCopyrightText: 2026 Echolot contributors
+# SPDX-License-Identifier: GPL-3.0-or-later
+# syntax=docker/dockerfile:1
+
+FROM golang:1.26-alpine AS build
+WORKDIR /src
+COPY go.mod ./
+RUN go mod download
+COPY . .
+ARG VERSION=dev
+RUN CGO_ENABLED=0 go build -trimpath \
+      -ldflags "-s -w -X main.Version=${VERSION}" \
+      -o /out/echolot-server ./cmd/echolot-server
+
+# Distroless static: no shell, no package manager; the server is pure Go.
+FROM gcr.io/distroless/static-debian12:nonroot
+COPY --from=build /out/echolot-server /echolot-server
+# State (device store + generated TLS) must persist across container restarts.
+ENV ECHOLOT_STATE_DIR=/state
+VOLUME ["/state"]
+# Ports are documentation only — run with network_mode: host (see compose.yaml):
+# the data plane must see real client source addresses/TTLs, and Docker's
+# userland NAT would falsify exactly what this server exists to observe.
+EXPOSE 8441/tcp 8442/udp 8443/tcp
+ENTRYPOINT ["/echolot-server"]