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/dns/Corefile

@@ -0,0 +1,16 @@
+# Authoritative fake zone for the e2e stand (plan C.4). Only e2e.test and its
+# subdomains are served — everything else is refused, which is fine: neither
+# the panel nor Postfix ever needs to resolve anything outside this zone here.
+#
+# The `file` plugin's own `reload` sub-directive (NOT the top-level `reload`
+# plugin, which only watches the Corefile itself) polls db.zone's mtime/serial
+# and reloads with no signal needed, so the test harness can rewrite the zone
+# file (to publish a domain's DKIM record) and just poll until the new record
+# is visible — no container restart, no sleep.
+e2e.test {
+    file /dns/db.zone {
+        reload 1s
+    }
+    log
+    errors
+}