milter, postfix, app, domain: cite the source of what they rely on (assumptions part B)

Comments only, no behaviour changed. go-milter v0.4.1 (session.go: one
session per connection, replaced after a non-continue reply; macros per
stage), Postfix 3.7.11 (milter8.c, smtpd_check.c
reject_auth_sender_login_mismatch(), mail_addr_find.c, showq_compat.c,
mail_conf_time.c, mail_params.h), Cyrus SASL 2.1.28 (saslpasswd.c,
plugins/sasldb.c: deleting a missing login is not an error), OpenDKIM
2.11.0~beta2 (opendkim-db.c, opendkim-genkey), Berkeley DB 5.3 db_dump,
rspamd 3.11.0. Comments that claimed what the source denies are corrected
(sasldb keeps the password as given, not hashed; an embedded newline is
kept, not truncated; rspamd sorts symbols by absolute weight); fixtures say
seen or composed. The sender check's From: parsing gets no citations: it is
replaced by the port of OpenDKIM's dkim_mail_parse next.

Contradictions left for their own commits: B-G5 cleanAddress, B-PS1 first
key wins, B-PQ1 the forced-expire flag, B-PQ10/PQ11 Postfix's time units.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/domain/dkim.go

@@ -37,6 +37,12 @@ func generateDKIMKey() (*rsa.PrivateKey, error) {
 // and entrypoint.sh); the parent directory carries setgid so the group is
 // inherited. The write is atomic (temp file + rename) so OpenDKIM never observes
 // a half-written key.
+//
+// OpenDKIM 2.11.0~beta2, libopendkim/dkim.c, dkim_privkey_load(): a key that
+// starts as PEM goes to OpenSSL's PEM_read_bio_PrivateKey(), which takes this
+// PKCS#1 form; opendkim/opendkim.c, dkimf_add_signrequest(): a key file
+// dkimf_securefile() finds insecure is only logged while RequireSafeKeys is no
+// (build/opendkim.conf).
 func writePrivateKeyPEM(path string, key *rsa.PrivateKey) error {
 	block := &pem.Block{
 		Type:  "RSA PRIVATE KEY",