feat: add --mode strip to remove the OCR text layer without rasterizing

Adds a processing mode that removes the invisible (render mode 3) OCR text
layer in place. Unlike `--ocr-engine none --force-ocr`, it does not
rasterize the page, so images and visible content are preserved unchanged
and the output is smaller rather than larger. Options that require
rasterization or OCR (--deskew, --clean, --sidecar, etc.) are rejected.

Only invisible text is removed; text drawn as visible glyphs under an
opaque image (some OCR engines, and OCRmyPDF v2.2 and earlier) cannot be
removed this way, as documented.

Closes #1435.

src/ocrmypdf/_validation.py

@@ -17,7 +17,7 @@ import pikepdf
 
 from ocrmypdf._defaults import DEFAULT_ROTATE_PAGES_THRESHOLD
 from ocrmypdf._exec import unpaper
-from ocrmypdf._options import OcrOptions
+from ocrmypdf._options import OcrOptions, ProcessingMode
 from ocrmypdf._plugin_manager import OcrmypdfPluginManager
 from ocrmypdf.exceptions import (
     BadArgsError,
@@ -118,8 +118,36 @@ def check_options_preprocessing(options: OcrOptions) -> None:
         )
 
 
+def check_options_strip(options: OcrOptions) -> None:
+    """Reject options that cannot apply in strip mode.
+
+    ``--mode strip`` removes the OCR text layer in place without rasterizing or
+    running OCR, so image-processing and OCR-output options have no effect.
+    """
+    if options.mode != ProcessingMode.strip_text:
+        return
+    incompatible = {
+        '--deskew': options.deskew,
+        '--clean': options.clean,
+        '--clean-final': options.clean_final,
+        '--remove-background': options.remove_background,
+        '--rotate-pages': options.rotate_pages,
+        '--oversample': options.oversample,
+        '--remove-vectors': options.remove_vectors,
+        '--sidecar': options.sidecar,
+    }
+    used = sorted(name for name, value in incompatible.items() if value)
+    if used:
+        raise BadArgsError(
+            "--mode strip removes the OCR text layer without rasterizing or "
+            "running OCR, so these options have no effect and are not allowed: "
+            f"{', '.join(used)}"
+        )
+
+
 def _check_plugin_invariant_options(options: OcrOptions) -> None:
     check_platform()
+    check_options_strip(options)
     check_options_sidecar(options)
     check_options_preprocessing(options)