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/api.py

@@ -328,6 +328,7 @@ def ocr(  # noqa: D417
     tesseract_oem: int | None = None,
     tesseract_thresholding: int | None = None,
     pdf_renderer: str | None = None,
+    rasterizer: str | None = None,
     tesseract_timeout: float | None = None,
     tesseract_non_ocr_timeout: float | None = None,
     tesseract_downsample_above: int | None = None,
@@ -481,6 +482,7 @@ def _pdf_to_hocr(  # noqa: D417
     tesseract_downsample_above: int | None = None,
     tesseract_downsample_large_images: bool | None = None,
     rotate_pages_threshold: float | None = None,
+    rasterizer: str | None = None,
     user_words: os.PathLike | None = None,
     user_patterns: os.PathLike | None = None,
     continue_on_soft_render_error: bool | None = None,