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/handlers.go

@@ -11,6 +11,9 @@ import (
 	"golang.org/x/crypto/bcrypt"
 )
 
+// draft-ietf-httpbis-rfc6265bis § 4.1.3.2: a cookie named __Host- is accepted
+// only with Secure, Path=/ and no Domain, from a secure origin - so no other
+// host can set or overwrite it, and clearing it needs Secure as well.
 const (
 	sessionCookieBase     = "selfpost_session"
 	sessionCookiePrefixed = "__Host-" + sessionCookieBase
@@ -46,6 +49,9 @@ func (m *Module) sessionToken(r *http.Request) (string, bool) {
 	case 1:
 		return token, true
 	default:
+		// RFC 6265 § 5.4: the Cookie header carries every cookie that
+		// matches, a parent domain's of the same name included. The prefixed
+		// name cannot be planted that way (see sessionCookiePrefixed).
 		logf("panel: %s %s carries %d cookies named %q — treating the request as signed out; "+
 			"another host on this domain is overwriting the session cookie, clear the cookies for the parent domain",
 			r.Method, r.URL.Path, n, name)
@@ -130,6 +136,9 @@ func (m *Module) submitLogin(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, "/", http.StatusSeeOther)
 }
 
+// setSessionCookie sets the session cookie. SameSite=Lax withholds it from a
+// cross-site request unless that is a top-level navigation with a safe method
+// (rfc6265bis § 5.8.3, § 5.6.7.1), so a cross-site POST arrives without it.
 func (m *Module) setSessionCookie(w http.ResponseWriter, token string) {
 	http.SetCookie(w, &http.Cookie{
 		Name:     m.sessionCookie(),