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/web/auth/session.go

@@ -31,7 +31,10 @@ func newSessionStore(st *store.Store, idle time.Duration) *sessionStore {
 
 // MaxAge is the session cookie's Max-Age in seconds, kept equal to the
 // sliding idle window so the browser drops the cookie no later than the
-// server would have expired it anyway.
+// server would have expired it anyway. RFC 6265 § 5.2.2: Max-Age sets the
+// expiry to now plus that many seconds, and a cookie set again under the same
+// name, domain and path replaces the old one (§ 5.3 step 11), so each renewal
+// slides it.
 func (s *sessionStore) MaxAge() int {
 	return int(s.idle.Seconds())
 }