feat: add --rasterizer CLI option to select PDF rasterization backend

Add user control over which rasterizer is used for PDF page rendering:
- 'auto' (default): prefers pypdfium when available, falls back to Ghostscript
- 'pypdfium': force pypdfium2 (errors if not installed)
- 'ghostscript': force traditional Ghostscript rasterizer

Changes:
- Add rasterizer field with validation to OCROptions model
- Add --rasterizer CLI argument in the Advanced options group
- Update rasterize_pdf_page hookspec to pass options to plugins
- Update pypdfium plugin with check_options hook for availability check
- Update both plugins to respect the rasterizer option

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/ocrmypdf/cli.py

@@ -373,6 +373,14 @@ Online documentation is located at:
         help="Choose OCR PDF renderer - the default option is to let OCRmyPDF "
         "choose.  See documentation for discussion.",
     )
+    advanced.add_argument(
+        '--rasterizer',
+        choices=['auto', 'ghostscript', 'pypdfium'],
+        default='auto',
+        help="Choose PDF page rasterizer. 'auto' prefers pypdfium when available, "
+        "falling back to Ghostscript. 'pypdfium' is faster but requires the "
+        "pypdfium2 package. 'ghostscript' uses the traditional Ghostscript rasterizer.",
+    )
     advanced.add_argument(
         '--rotate-pages-threshold',
         default=DEFAULT_ROTATE_PAGES_THRESHOLD,