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>

internal/backup/backup.go

@@ -88,7 +88,9 @@ type Params struct {
 // copy taken then can hold a message twice or not at all, so what it would
 // bring back is not the queue. Nor could the panel read it - Postfix keeps
 // every queue file readable by its own user alone, which is why the panel's
-// backup failed outright from 1.3.0 to 2.0.0. What is not carried over is mail
+// backup failed outright from 1.3.0 to 2.0.0 (date not recorded; Postfix
+// 3.7.11, conf/postfix-files: active, deferred, incoming and the other queue
+// directories are $mail_owner 700). What is not carried over is mail
 // still waiting for delivery; a start without a queue directory builds an
 // empty one. Whoever needs the queue stops the instance and archives the
 // project directory (guide § Taking a full backup).
@@ -380,7 +382,11 @@ func writeTarFile(tw *tar.Writer, name string, mode fs.FileMode, srcPath string)
 // snapshotDB produces a consistent copy of the SQLite database at dbPath using
 // VACUUM INTO, so the backup captures a coherent point-in-time image even while
 // the panel is writing to the live file under WAL. It returns the snapshot path
-// and a cleanup function the caller must defer.
+// and a cleanup function the caller must defer. (sqlite.org/lang_vacuum.html,
+// "VACUUM INTO": "transactional in the sense that the generated output
+// database is a consistent snapshot"; modernc.org/sqlite v1.60.1, sqlite.go,
+// applyQueryParams(): each _pragma is run as "pragma <value>" on connect,
+// busy_timeout first.)
 func snapshotDB(dbPath string) (path string, cleanup func(), err error) {
 	dir, err := os.MkdirTemp("", "selfpost-backup-")
 	if err != nil {