fix(B04): prevent image URI flash across initial render and detail remounts

scripts/image-renderer-regression.tsx

@@ -103,7 +103,21 @@ function testMarkdownImageServerFallback(): void {
 
   const source = 'C:\\workspace\\source-only.png'
   const sourceFallback = renderToStaticMarkup(<MarkdownImage src={source} urlList={[]} />)
-  assert.ok(sourceFallback.includes(source))
+  assert.ok(!sourceFallback.includes(source), 'unresolved source is never fallback text')
+  assert.ok(sourceFallback.length < 500)
+
+  const synthetic = 'data:image/png;base64,' + 'A'.repeat(24000)
+  const first = renderToStaticMarkup(<MarkdownImage src={synthetic} alt="" urlList={[synthetic]} />)
+  assert.match(first, /<img /)
+  assert.ok(first.includes(`src="${synthetic}"`))
+  assert.ok(!first.includes('cr-image-block-fallback'))
+  assert.ok(!first.replace(/<[^>]*>/g, '').includes('data:image'))
+  for (const invalid of ['javascript:alert(1)', 'data:text/html,' + 'A'.repeat(24000)]) {
+    const markup = renderToStaticMarkup(<MarkdownImage src={invalid} alt="" urlList={[]} />)
+    assert.ok(markup.length < 500)
+    assert.ok(!markup.includes(invalid))
+    assert.ok(!markup.includes('<img '))
+  }
 }
 
 function testPreviewDialogServerStructure(): void {