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/process_check.go

@@ -65,7 +65,10 @@ func parseSupervisorStatus(out string) map[string]string {
 
 // checkInboundRelayOff asserts the default image does not accept mail on
 // port 25: smtp/inet is absent from master.cf (Debian's stock listener is
-// removed when INBOUND_RELAY_ENABLE is not true).
+// removed when INBOUND_RELAY_ENABLE is not true). For an absent service
+// postconf -M prints nothing on stdout and "warning: unmatched request" on
+// stderr (Postfix 3.7.11, src/postconf/postconf_master.c,
+// pcf_show_master_entries()).
 func checkInboundRelayOff(s *stack) error {
 	out, err := s.execIn("selfpost", "postconf", "-M", "smtp/inet")
 	combined := out