feat: implement C.4 — hermetic container e2e gate + native release build

Separate test/e2e Go module drives the shipped deploy/docker-compose.yml
(plus a test-only override: self-signed cert, low ports, isolated compose
project) against a fake DNS zone (CoreDNS) and an smtp-sink MX, exactly as
an administrator and their applications would over HTTP/SMTP — covering the
class of failure unit tests can't see (container wiring). Positive path:
setup -> login -> domain -> DKIM record published into the fake zone ->
application -> SMTP AUTH send -> DKIM verified against the DNS-published
key -> send-log queued->sent. Negative: no-AUTH/unauthenticated relay,
sender/login mismatch, L1 (anvil) and L2 (panel) rate limits, journal-milter
fail-open, SELFPOST_HOSTNAME gate, session survives docker restart.

release.yml moves off qemu to a native per-arch build (amd64/arm64), each
gated by this suite before its tag is pushed and merged into the version
manifest.

Verified green on selfpost.mixfed.ru via `make e2e`; go vet/gofmt clean in
both modules.

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

test/e2e/sink/Dockerfile

@@ -0,0 +1,26 @@
+# SMTP sink-MX for the e2e stand (plan C.4): accepts any mail on 25 and dumps
+# each transaction to its own file under /mail, so the test harness can read
+# the raw received message (headers + body, including the DKIM-Signature
+# Postfix/OpenDKIM added) straight off disk. smtp-sink ships in the postfix
+# package itself — no new dependency.
+FROM debian:bookworm-slim
+
+ENV DEBIAN_FRONTEND=noninteractive
+RUN echo "postfix postfix/mailname string localhost" | debconf-set-selections \
+ && echo "postfix postfix/main_mailer_type string Internet Site" | debconf-set-selections \
+ && apt-get update \
+ && apt-get install -y --no-install-recommends postfix \
+ && rm -rf /var/lib/apt/lists/*
+
+RUN mkdir -p /mail
+WORKDIR /mail
+
+# -d: one file per transaction (a pseudo-random suffix is appended
+# automatically), so the harness just watches the directory for a new file
+# instead of parsing a shared multi-message dump. Binding :25 needs root, and
+# smtp-sink refuses to run as root without an explicit -u to switch to after
+# binding — "-u root" satisfies that check while staying root throughout,
+# which is fine here: throwaway container, isolated e2e network, never
+# exposed.
+ENTRYPOINT ["/usr/sbin/smtp-sink"]
+CMD ["-u", "root", "-d", "/mail/%Y%m%d%H%M%S.", "0.0.0.0:25", "100"]