Reject non-canonical timestamps in Timestamp.Scan

time.Parse accepts a comma before fractional seconds, so Scan took
"2026-10-03T12:37:05,123Z" and Value then wrote it back with a period.
Scan now requires the parsed instant to re-format byte-identically to
its input, and the rejection test covers the comma form.

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

internal/store/time.go

@@ -47,6 +47,9 @@ func (t *Timestamp) Scan(src any) error {
 	if err != nil {
 		return fmt.Errorf("store: scan Timestamp %q: %w", s, err)
 	}
+	if parsed.Format(timestampLayout) != s {
+		return fmt.Errorf("store: scan Timestamp %q: want %s", s, timestampLayout)
+	}
 	t.Time = parsed
 	return nil
 }