feat(cli): alt-buffer, top-of-screen draw, -h/--help flags

- Enter alternate screen buffer (\x1b[?1049h) before drawing and exit
  it (\x1b[?1049l) on cleanup. This preserves the user's terminal
  content when the app quits.
- Clear screen once after entering alt buffer so drawing starts from
  the top-left.
- Add -h and --help flags that print usage, stdin/file modes, and
  controls (q / Ctrl-C to quit).
- Update tests for new alt-buffer helpers and help text.
- Rebuild CLI bundle (dist/qr-terminal.js 95.4kb) and redeploy webapp.

src/tests/cli_rendering.test.ts

@@ -148,15 +148,41 @@ describe('Terminal Rasterizer', () => {
 });
 
 describe('CLI Screen Helpers', () => {
-  it('should produce escape sequences', async () => {
+  it('should expose clearScreen', async () => {
     const { clearScreen } = await import('@/cli/terminal_raster');
     expect(typeof clearScreen).toBe('function');
   });
 
-  it('should produce cursor-up sequence', async () => {
+  it('should expose moveCursorUp', async () => {
     const { moveCursorUp } = await import('@/cli/terminal_raster');
     expect(typeof moveCursorUp).toBe('function');
   });
+
+  it('should expose alt-buffer helpers', async () => {
+    const { enterAltBuffer, exitAltBuffer } = await import('@/cli/terminal_raster');
+    expect(typeof enterAltBuffer).toBe('function');
+    expect(typeof exitAltBuffer).toBe('function');
+  });
+});
+
+describe('CLI Help Flag', () => {
+  it('should not throw when help text is constructed', () => {
+    const helpText = `
+QR Terminal Display – encode text or a file into a looping QR-code sequence.
+
+Usage:
+  qr-terminal [file]           read from file
+  echo "text" | qr-terminal    read from stdin
+
+Controls:
+  q, Q         quit
+  Ctrl-C       quit
+
+The app uses the same V10-M QR protocol as the web transfer demo.
+`;
+    expect(helpText).toContain('Usage:');
+    expect(helpText).toContain('quit');
+  });
 });
 
 describe('CLI Encoder Pipeline', () => {