Remove lossy JBIG2 support, retain lossless JBIG2 only

Lossy JBIG2 has been removed due to well-documented risks of character
substitution errors (e.g., 6/8 confusion). The --jbig2-lossy and
--jbig2-page-group-size arguments are now deprecated and ignored with
a warning.

Changes:
- Remove jbig2_lossy and jbig2_page_group_size from OCROptions
- Simplify optimize.py to use single-image JBIG2 encoding only
  (no symbol dictionaries/JBIG2Globals)
- Remove convert_group() from jbig2enc.py
- Deprecate CLI args with warnings for backward compatibility
- Update documentation to explain lossless-only JBIG2

tests/test_optimize.py

@@ -81,8 +81,8 @@ def test_jpg_png_params(resources, outpdf):
 
 
 @needs_jbig2enc
-@pytest.mark.parametrize('lossy', [False, True])
-def test_jbig2_lossy(lossy, resources, outpdf):
+def test_jbig2_lossless(resources, outpdf):
+    """Test that JBIG2 lossless encoding works without JBIG2Globals."""
     args = [
         resources / 'ccitt.pdf',
         outpdf,
@@ -99,19 +99,14 @@ def test_jbig2_lossy(lossy, resources, outpdf):
         '--jbig2-threshold',
         '0.7',
     ]
-    if lossy:
-        args.append('--jbig2-lossy')
 
     check_ocrmypdf(*args)
 
     with pikepdf.open(outpdf) as pdf:
         pim = pikepdf.PdfImage(next(iter(pdf.pages[0].images.values())))
         assert pim.filters[0] == '/JBIG2Decode'
-
-        if lossy:
-            assert '/JBIG2Globals' in pim.decode_parms[0]
-        else:
-            assert len(pim.decode_parms) == 0
+        # Lossless JBIG2 has no JBIG2Globals (no shared symbol dictionary)
+        assert len(pim.decode_parms) == 0
 
 
 @needs_pngquant