fix(media): tighten hasSingleByteTextShape to reject mixed ASCII/high-byte blobs

Raise the ASCII floor to 70% and add an explicit 30% high-byte cap.
The previous 50% threshold accepted alternating 0x41/0xFF buffers
(50% ASCII, 0 control bytes), which decoded through Latin-1 and passed
the printable-ratio gate — allowing opaque binary data to slip through
as a CSV or Markdown document.

Real single-byte text exports (e.g. Excel Latin-1 CSVs with accented
chars like é, ñ) rarely exceed 20-25% high bytes, so the tighter
thresholds do not regress legitimate input.

Adds a regression test: 9000 bytes alternating 'A'/0xFF must be
rejected as path-not-allowed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/media/web-media.test.ts

@@ -332,6 +332,31 @@ describe("loadWebMedia", () => {
     },
   );
 
+  it.each([
+    { label: "CSV", fileName: "alternating-high.csv" },
+    { label: "Markdown", fileName: "alternating-high.md" },
+  ])("rejects alternating ASCII/high-byte data disguised as %s", async ({ fileName }) => {
+    const fakeTextFile = path.join(fixtureRoot, fileName);
+    // Alternating 0x41 ('A') and 0xFF — exactly 50% ASCII, 50% high bytes.
+    // With the old 50% threshold hasSingleByteTextShape would accept this;
+    // the tightened 70%/30% thresholds must reject it.
+    const mixed = Buffer.alloc(9000);
+    for (let i = 0; i < mixed.length; i += 1) {
+      mixed[i] = i % 2 === 0 ? 0x41 : 0xff;
+    }
+    await fs.writeFile(fakeTextFile, mixed);
+    await expect(
+      loadWebMedia(fakeTextFile, {
+        maxBytes: 1024 * 1024,
+        localRoots: "any",
+        readFile: async (filePath) => await fs.readFile(filePath),
+        hostReadCapability: true,
+      }),
+    ).rejects.toMatchObject({
+      code: "path-not-allowed",
+    });
+  });
+
   it.each([
     { label: "CSV", fileName: "high-bytes.csv" },
     { label: "Markdown", fileName: "high-bytes.md" },

src/media/web-media.ts

@@ -164,7 +164,8 @@ function hasSingleByteTextShape(buffer: Buffer): boolean {
     }
   }
   const total = buffer.length;
-  return control === 0 && asciiText / total >= 0.5;
+  const highBytes = total - asciiText - control;
+  return control === 0 && asciiText / total >= 0.7 && highBytes / total <= 0.3;
 }
 
 function decodeHostReadText(buffer: Buffer): string | undefined {