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

docs/jbig2.md

@@ -43,33 +43,21 @@ be required depending on your system.
 [sudo] apt install autotools-dev automake libtool libleptonica-dev pkg-config
 :::
 
-{#jbig2-lossy}
+## JBIG2 Compression
 
-## Lossy mode JBIG2
+OCRmyPDF uses JBIG2 lossless compression for bitonal (black and white)
+images. This provides excellent compression ratios compared to the older
+CCITT G4 standard, while preserving the exact pixel content of the
+original image.
 
-OCRmyPDF provides lossy mode JBIG2 as an advanced and potentially
-dangerous feature. Users should [review the technical concerns with
-JBIG2 in lossy mode](https://en.wikipedia.org/wiki/JBIG2#Disadvantages)
-and decide if this feature is acceptable for their use case. In general,
-this mode should not be used for archival purposes, should not be used
-when the original document is not available or will be destroyed, and
-should not be used when numbers present in the document are important,
-because there is a risk of 6/8 and 8/6 substitution errors.
+You can adjust the threshold for JBIG2 compression with
+`--jbig2-threshold`. The default is 0.85.
 
-JBIG2 lossy mode does achieve higher compression ratios than any other
-monochrome (bitonal) compression technology; for large text documents
-the savings are considerable. JBIG2 lossless still gives great
-compression ratios and is a major improvement over the older CCITT G4
-standard.
-
-To turn on JBIG2 lossy mode, add the argument `--jbig2-lossy`.
-`--optimize {1,2,3}` are necessary for the argument to take effect also
-required. Also, a JBIG2 encoder must be installed as described in the
-previous section.
-
-You can adjust the threshold for JBIG2 compression with the
-`--jbig2-threshold`. The default is 0.85, meaning that if two symbols
-are 85% similar, they will be compressed together.
-
-*Due to an oversight, ocrmypdf v7.0 and v7.1 used lossy mode by
-default.*
+:::{note}
+Previous versions of OCRmyPDF supported a lossy JBIG2 mode
+(`--jbig2-lossy`). This feature has been removed due to the well-known
+risk of character substitution errors (e.g., 6/8 confusion). See
+[JBIG2 disadvantages](https://en.wikipedia.org/wiki/JBIG2#Disadvantages)
+for more information on why lossy JBIG2 is problematic. The `--jbig2-lossy`
+and `--jbig2-page-group-size` arguments are now ignored with a warning.
+:::

docs/optimizer.md

@@ -28,9 +28,6 @@ header-rows: 1
   - Enables lossless optimizations, such as transcoding images to more
       efficient formats. Also compress other uncompressed objects in the
       PDF and enables the more efficient "object streams" within the PDF.
-      (If ``--jbig2-lossy`` is issued, then lossy JBIG2 optimization is used.
-      The decision to use lossy JBIG2 is separate from standard optimization
-      settings.)
 * - ``--optimize 2``
   - ``-O2``
   - All of the above, and enables lossy optimizations and color quantization.
@@ -105,7 +102,3 @@ quality image may be suitable for storage after OCR.
 
 It is not possible to optimize all image types. Uncommon image types may
 be skipped by the optimizer.
-
-OCRmyPDF provides `lossy mode JBIG2 <jbig2-lossy>`{.interpreted-text
-role="ref"} as an advanced feature that additional requires the argument
-`--jbig2-lossy`.