feat(injection): E3 — rot13 layer for comment-block injection

Adds rot13 to the variantSet built in scanForInjection(), so
imperative phrases hidden as rot13 inside code comments still hit
the existing CRITICAL/HIGH/MEDIUM pattern arrays.

normalizeForScan() already covers base64, hex, URL, and HTML decoding
in a 3-iteration loop — those are NOT duplicated here. rot13 is the
only genuinely new variant: it is its own inverse and not part of any
NIST/Unicode normalization spec, so it has to be applied explicitly.

Threshold: only inputs >40 chars enter the rot13 pass, to suppress
false positives on accidental letter-shifts in tokens, ids, and short
identifiers. Variants are deduplicated against the existing set so
matchers do not run twice.

3 new tests in injection-patterns.test.mjs (rot13 detection, sub-40
char suppression, plaintext path still green). Total 168 tests pass.

Closes E3 in critical-review-2026-04-20.md.

plugins/llm-security/scanners/lib/string-utils.mjs

@@ -459,6 +459,30 @@ const HOMOGLYPH_MAP = Object.freeze({
  * @param {string} s
  * @returns {string}
  */
+/**
+ * Apply rot13 (Caesar shift by 13) to ASCII letters.
+ * Non-letters pass through unchanged. The transform is its own inverse.
+ *
+ * Used by E3 comment-block injection detection: attackers sometimes hide
+ * imperative phrases ("ignore previous instructions") in rot13 inside
+ * code comments. normalizeForScan() does not apply rot13, so this layer
+ * is added explicitly to the variantSet in scanForInjection().
+ *
+ * @param {string} s
+ * @returns {string}
+ */
+export function rot13(s) {
+  if (!s) return s;
+  let out = '';
+  for (let i = 0; i < s.length; i++) {
+    const c = s.charCodeAt(i);
+    if (c >= 65 && c <= 90) out += String.fromCharCode(((c - 65 + 13) % 26) + 65);
+    else if (c >= 97 && c <= 122) out += String.fromCharCode(((c - 97 + 13) % 26) + 97);
+    else out += s[i];
+  }
+  return out;
+}
+
 export function foldHomoglyphs(s) {
   if (!s) return s;
   // Fast path: pure ASCII has nothing to fold and NFKC is identity.