test: comprehensive content security tests (47 tests)

Covers all 4 defense layers:
- Datamarking: marker format, session consistency, text-only application
- Content envelope: wrapping, ZWSP marker escaping, filter warnings
- Content filter hooks: URL blocklist, custom filters, warn/block modes
- Instruction block: SECURITY section content, ordering, generation
- Centralized wrapping: source-level verification of integration
- Chain security: recursion guard, rate-limit exemption, activity suppression
- Hidden element stripping: 7 CSS techniques, ARIA injection, false positives
- Snapshot split format: scoped vs root output, resume integration

Also fixes: visibility:hidden detection, case-insensitive ARIA pattern matching.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

browse/src/content-security.ts

@@ -126,6 +126,11 @@ export async function markHiddenElements(page: Page | Frame): Promise<string[]>
           isHidden = true;
           reason = 'clip hiding';
         }
+        // Check visibility: hidden
+        else if (style.visibility === 'hidden') {
+          isHidden = true;
+          reason = 'visibility hidden';
+        }
 
         if (isHidden) {
           el.setAttribute('data-gstack-hidden', 'true');
@@ -143,7 +148,7 @@ export async function markHiddenElements(page: Page | Frame): Promise<string[]>
 
         if (labelText) {
           for (const pattern of ariaPatterns) {
-            if (new RegExp(pattern).test(labelText)) {
+            if (new RegExp(pattern, 'i').test(labelText)) {
               el.setAttribute('data-gstack-hidden', 'true');
               found.push(`[${el.tagName.toLowerCase()}] ARIA injection: "${labelText.slice(0, 60)}..."`);
               break;