M11: PDF Optimizer stores black-and-white images as CCITT Group 4 when smaller

pdfcraft_filters::encode_g4 encodes packed 1-bit rows as Group 4 (ITU-T
T.6, /K -1) through the fax crate (MIT, already in the tree via tiff),
with rows capped at 32,767 samples so its 16-bit positions can't
overflow; round trips are checked against hayro-ccitt.

The optimizer (Settings::mono, CCITT G4 by default, so Reduce File Size
too) re-encodes a 1-bit image of one colour component, or an image
mask, stored with no filter or general-purpose ones, when that is
smaller as stored. The samples decode to the same bits, so colour
space, decode array and masks stay. PDF Optimizer gets a Monochrome
Images row and doc_optimize a mono option. optimize.mono-compression
goes from planned to partial (JBIG2 and monochrome downsampling not
yet).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BfMGHQNgdq5yf5tKZHGR3z

parity/acrobat-features.toml

@@ -7325,7 +7325,7 @@ status = "shipped"
 commands = ["optimize.reduce"]
 tools = ["doc_reduce"]
 evidence = ["crates/ui-egui/tests/it/create.rs::reduce_file_size_writes_a_compact_copy", "crates/ui-egui/tests/it/create.rs::reduce_file_size_saves_nothing_when_the_file_is_already_small", "crates/engine/src/tests.rs::create_and_reduce", "crates/automation/tests/it/automation.rs::creating_and_reducing_through_tools", "crates/automation/tests/it/automation.rs::optimizing_through_tools", "crates/optimize/tests/optimize.rs::an_identity_decode_array_does_not_stop_reduce"]
-notes = "images above 225 ppi to 150 ppi JPEG medium (after merging page pictures printed as one-pixel strips into one image), thumbnails dropped, resources merged, object streams; scanner images with the default decode array are included; a copy that isn't smaller is not saved (\"already as small as it can be made\"); runs in the background with a progress bar and Cancel"
+notes = "images above 225 ppi to 150 ppi JPEG medium (after merging page pictures printed as one-pixel strips into one image), black-and-white images to CCITT Group 4 when that is smaller, thumbnails dropped, resources merged, object streams; scanner images with the default decode array are included; a copy that isn't smaller is not saved (\"already as small as it can be made\"); runs in the background with a progress bar and Cancel"
 
 [[feature]]
 id = "optimize.compatibility-level"
@@ -7381,11 +7381,11 @@ title = "Monochrome compression (JBIG2, CCITT G4)"
 area = "K"
 tier = "P1"
 milestone = "M11"
-status = "planned"
-commands = []
-tools = []
-evidence = []
-notes = ""
+status = "partial"
+commands = ["optimize.advanced", "optimize.reduce"]
+tools = ["doc_optimize", "doc_reduce"]
+evidence = ["crates/filters/src/ccitt.rs::bitmaps_round_trip", "crates/filters/src/ccitt.rs::a_page_of_text_like_marks_round_trips_and_shrinks", "crates/filters/src/ccitt.rs::what_it_cannot_encode_is_an_error", "crates/optimize/tests/optimize.rs::black_and_white_images_become_ccitt_group_4", "crates/automation/tests/it/automation.rs::doc_optimize_stores_black_and_white_images_as_ccitt_group_4", "crates/ui-egui/tests/it/create.rs::the_pdf_optimizer_dialog_saves_an_optimized_copy"]
+notes = "CCITT Group 4, lossless: PDF Optimizer ▸ Images ▸ Monochrome Images (on by default, so Reduce File Size does it too; doc_optimize mono) stores a 1-bit image of one colour component (grey, indexed, a separation) or an image mask, kept with no filter or general-purpose ones, as Group 4 when that is smaller as stored; the samples decode to the same bits, so colour space, decode array and masks stay. Group 4 wins on scans; on clean computer-drawn 1-bit art Flate can be smaller, and the image then stays. Not yet: JBIG2 (lossless or lossy, which Adobe's help says beats CCITT for monochrome), monochrome downsampling, inline images, soft masks and /Mask images, rows wider than 32,767 samples."
 
 [[feature]]
 id = "optimize.only-if-smaller"