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/builtin_plugins/ghostscript.py

@@ -128,8 +128,13 @@ def rasterize_pdf_page(
     rotation,
     filter_vector,
     stop_on_soft_error,
+    options=None,
 ):
     """Rasterize a single page of a PDF file using Ghostscript."""
+    # Check if user explicitly requested a different rasterizer
+    if options is not None and options.rasterizer == 'pypdfium':
+        return None  # Let pypdfium handle it (it will error in check_options if unavailable)
+
     ghostscript.rasterize_pdf(
         input_file,
         output_file,