chore/docs: move to GitHub as the single home; drop archived-spec references

Codeberg is being retired as the project's public site, so every reference now
points at GitHub. That includes the Go module path (codeberg.org/mix/selfpost →
github.com/mixeme/selfpost): leaving an import path on a host that is going
away would break `go get` and `go install`, so this is not only a docs change.
Touches go.mod, test/e2e/go.mod, all imports, Makefile MODULE, the -ldflags
version stamp in build/Dockerfile and docs/development.md, the licence headers
in the SVG/HTML assets, and README (no more primary/mirror pair).

Comments no longer cite the archived specification. "spec 7.6.1", "spec 5.1"
and friends pointed into docs/archive/specification-v1.0.md, which is marked as
not a source of truth; each is now a reference to the live document that owns
the subject — architecture.md (with section), product.md, security.md or the
README. The review only asked for the 7.x refs (code-review.md § 4), but 4/5/6/
8/9 had the same defect, so they went too. Comments only, no behaviour change.

Also closes the remaining review items: architecture.md gained a Code layers
section with the layer diagram (A2), and TestParseDelivery gained the exotic
mail.log cases (§ 3).

Fixes a bug that last test found: the delivery-line pattern matched status=
greedily, taking the *last* occurrence on the line. Postfix appends the remote
server's reply verbatim, so a rejection whose reply quoted "status=sent" was
filed as a delivered message in the send log. It now takes the first status=
after the recipient, which is the real field.

R7 (CONTRIBUTING.md) moved to roadmap 2.x — one developer, no external PR flow,
so the file would have no audience yet. R1 (compose image tag) and the git tag
stay in roadmap § v1.x as the release-commit steps.

gofmt/go vet clean on both modules; go test ./... green except the three known
Windows-only failures (file perms, backslash paths, renaming an open file).
Not exercised on the dev server — no Docker locally.

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

test/e2e/negative_test.go

@@ -68,8 +68,8 @@ func testLevel2RateLimit(t *testing.T, sc *scenario) {
 }
 
 // testSenderLoginMismatch is plan C.4 negative check 2: an authenticated
-// application cannot send as a sender it does not own (spec 5.1 p.3,
-// reject_sender_login_mismatch) — the core anti-spoofing control.
+// application cannot send as a sender it does not own (architecture.md § Mail
+// path, reject_sender_login_mismatch) — the core anti-spoofing control.
 func testSenderLoginMismatch(t *testing.T, sc *scenario) {
 	res := attemptSend(sendAttempt{
 		authLogin: sc.appLogin, authPassword: sc.appPassword,
@@ -118,10 +118,10 @@ func testForeignRelayRejected(t *testing.T, sc *scenario) {
 }
 
 // testJournalMilterFailOpen is plan C.4 negative check 6: the journal-milter
-// is monitoring-only and fails open (spec 7.3) — stopping the panel process
-// (which owns the milter socket) must not block mail, and must not crash the
-// container (crashexit only fires on PROCESS_STATE_FATAL, a clean supervisor
-// stop is STOPPED, see build/crashexit.py).
+// is monitoring-only and fails open (architecture.md § Mail path) — stopping
+// the panel process (which owns the milter socket) must not block mail, and
+// must not crash the container (crashexit only fires on PROCESS_STATE_FATAL, a
+// clean supervisor stop is STOPPED, see build/crashexit.py).
 func testJournalMilterFailOpen(t *testing.T, sc *scenario) {
 	if _, err := h.execIn("selfpost", "supervisorctl", "-c", "/etc/supervisor/supervisord.conf", "stop", "panel"); err != nil {
 		t.Fatalf("stop panel: %v", err)
@@ -196,12 +196,12 @@ func testSessionSurvivesRestart(t *testing.T, sc *scenario) {
 }
 
 // testLevel1RateLimit is plan C.4 negative check 4: the native Postfix anvil
-// backstop (smtpd_client_message_rate_limit, spec 5 p.5), set by the override
-// to RATE_LIMIT_MESSAGES_PER_IP=50, rejects once exceeded. It uses a dedicated
-// application with no level-2 limit of its own, and retries well past that
-// count, so the result is unambiguous regardless of how much of the shared
-// per-IP budget earlier subtests already spent (they stay well under 50
-// between them).
+// backstop (smtpd_client_message_rate_limit, README § Rate limiting), set by
+// the override to RATE_LIMIT_MESSAGES_PER_IP=50, rejects once exceeded. It
+// uses a dedicated application with no level-2 limit of its own, and retries
+// well past that count, so the result is unambiguous regardless of how much of
+// the shared per-IP budget earlier subtests already spent (they stay well
+// under 50 between them).
 func testLevel1RateLimit(t *testing.T, sc *scenario) {
 	login, password, err := sc.panel.addApplication(sc.domainID, "l1app", "wildcard", "")
 	if err != nil {