store, web: cite the source of what they rely on (assumptions part D)

Comments only, no behaviour changed. modernc.org/sqlite v1.60.1 (the
_pragma list on every connection, busy_timeout first; extended result
codes; a deferred BEGIN; multi-statement Exec) and SQLite 3.53.4's
documentation (transactions, WAL, foreign keys, NOCASE, upsert, julianday,
partial indexes); htmx 2.0.4 src/htmx.js; the Fetch and HTML standards for
Sec-Fetch-Site, Origin, SameSite, __Host-, CSP and defer order; Go's
html/template. Comments the source contradicts are corrected - the driver
does not serialise writes (SetMaxOpenConns(1) does), htmx:load fires per
swapped element, no-store does not keep a page out of history.

Left for their own commits: D-S3 a migration two processes can both run,
D-S20 recipient case, D-B3 Origin: null under Referrer-Policy: no-referrer,
D-H5/H6/H7/H9 (audit M11, stage 4).

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

internal/store/dmarc.go

@@ -80,8 +80,10 @@ type DMARCIngestStats struct {
 }
 
 // InsertDMARCReport stores a parsed report and its records, replacing any prior
-// row with the same reporter/report_id/domain triple.
+// row with the same reporter/report_id/domain triple; the prior report's
+// records go with it by ON DELETE CASCADE (see DeleteDomain).
 func (s *Store) InsertDMARCReport(rep DMARCReport) (int64, error) {
+	// The first statement is a write (see UpdateStatus).
 	tx, err := s.db.Begin()
 	if err != nil {
 		return 0, fmt.Errorf("begin insert dmarc report: %w", err)