build, deploy, inbound, dmarc, backup, e2e: cite the source of what they rely on (assumptions part C)

Comments only, no behaviour changed. Each main.cf/master.cf parameter the
build sets, the wrapper's and the reload hook's reliance on Postfix 3.7.11
(postfix-script, master.5, postconf.5, cleanup_init.c, smtpd_peer.c,
README_FILES), supervisor 4.2.5 (process.py, options.py, events.rst),
logrotate 3.21.0, OpenDKIM's config, Cyrus SASL's sasldb, rspamd 3.11.1
(milter_headers.lua, multimap, the map watcher), Docker Compose, CoreDNS
and certbot now say where that is written. Measurements already there stay
measurements ("date not recorded" where none was); hand-composed fixtures
say so.

Contradictions left for their own commits: C-P31 cleanup's size limit,
C-P42 master outside supervisor's signal, C-V5/V6 startsecs, C-V10,
C-S3/Q3 sasldb written in place, C-Q4/A1 certbot's symlinks, C-R7 rspamd
skips local clients but the signature routine does not, C-P8, C-R4, C-R12,
C-M7, C-E2, C-K8/K9, C-D1.

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

test/e2e/dns_zone.go

@@ -48,7 +48,9 @@ func writeZone(stageDir string, extra []txtRecord) error {
 	fmt.Fprintf(&b, "ns      IN A    10.77.0.10\n")
 	// The sink-MX: Postfix's outbound delivery for the recipient domain used by
 	// every positive-path send resolves here via the domain's implicit MX
-	// fallback to its own A record (RFC 5321) — no explicit MX record needed.
+	// fallback to its own A record (RFC 5321 § 5.1: with no MX records the
+	// domain is treated as having an implicit MX of itself) — no explicit MX
+	// record needed.
 	fmt.Fprintf(&b, "sink    IN A    10.77.0.11\n")
 	for _, r := range extra {
 		fmt.Fprintf(&b, "%s IN TXT %s\n", r.name, chunkTXT(r.value))
@@ -57,7 +59,8 @@ func writeZone(stageDir string, extra []txtRecord) error {
 }
 
 // chunkTXT splits a TXT value into <255-byte quoted strings (the DNS
-// <character-string> limit), which resolvers concatenate back into one value.
+// <character-string> limit, RFC 1035 § 3.3), which a DKIM verifier
+// concatenates back into one value (RFC 6376 § 3.6.2.2).
 // A DKIM RSA-2048 public key's base64 comfortably exceeds 255 bytes on its
 // own, so this is required, not defensive.
 func chunkTXT(value string) string {