M11: an indexed image's palette says which of its samples is dark, so it is averaged too

#5426 left indexed images at their resolution rather than guess which sample is
dark. The palette says: dark_is_one now maps samples 0 and 1 through the decode array
to palette entries (rounded and clipped to hival, ISO 32000-2 §8.9.5.2) and compares
how light those entries are (`lightness`: the grey, or Rec. 601 luma over DeviceRGB).
Only DeviceGray and DeviceRGB bases count, the table a string or a stream (within
4 KiB), directly or by reference: there the palette's values are the colour, and
under /DefaultGray or /DefaultRGB no polarity is asked for. Other bases (CMYK,
calibrated, ICC-based, a resource name), two entries alike, an entry the table
doesn't hold or a hival past 255 still say nothing; separations and DeviceN still
aren't averaged, their tint transforms unread. #5426's tone guard applies as to any
image.

#5426's indexed thin-rule test now checks what it was for: with the palette either
way round and the decode array either way, the averaged image holds the averaged
DeviceGray one's samples (flipped where its samples are) and renders the same. A unit
test covers each base, both orders, an inverting decode array, the weights of luma,
the table by reference and in a stream, and every refusal; the older polarity test's
doc says what is known now.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

parity/acrobat-features.toml

@@ -7384,8 +7384,8 @@ milestone = "M11"
 status = "partial"
 commands = ["optimize.advanced"]
 tools = ["doc_optimize"]
-evidence = ["crates/optimize/tests/optimize.rs::images_are_measured_where_drawn_and_downsampled", "crates/optimize/tests/optimize.rs::settings_choose_what_happens", "crates/automation/tests/it/automation.rs::optimizing_through_tools", "crates/optimize/tests/optimize.rs::a_page_picture_printed_as_one_pixel_strips_is_merged_into_one_image", "crates/optimize/tests/optimize.rs::images_that_do_not_tile_a_picture_are_left_alone", "crates/optimize/tests/optimize.rs::a_deeply_nested_form_is_measured_at_its_largest_use", "crates/optimize/tests/optimize.rs::images_are_not_resampled_when_some_uses_could_not_be_measured", "crates/optimize/tests/optimize.rs::image_resolution_uses_the_pages_physical_unit", "crates/optimize/tests/optimize.rs::optimization_retains_an_image_below_threshold_on_a_scaled_page", "crates/optimize/tests/optimize.rs::shared_forms_are_measured_in_each_pages_physical_units", "crates/optimize/tests/optimize.rs::ambiguous_monochrome_text_is_recompressed_without_lossy_reduction", "crates/optimize/src/images.rs::it_matches_the_slow_way", "crates/automation/tests/it/automation.rs::doc_optimize_averages_black_and_white_scans_down", "crates/ui-egui/tests/it/create.rs::the_pdf_optimizer_dialog_saves_an_optimized_copy"]
-notes = "colour and grayscale, bicubic, effective resolution measured where drawn (every use, with page-local UserUnit scaling, through nested forms up to 12 deep; when a use can't be measured, nothing is resampled); image fragments stacked edge to edge are merged first (side-by-side tiles not yet); monochrome grey images and image masks (1-bit, CCITT fax, and eight-bit black-and-white grey and RGB packed to one bit) averaged by area to 300 ppi above 450 ppi by default, a tie to the dark sample (black in grey, painted in a mask, after a decode array of two different numbers), then stored as Group 4 or 1-bit Flate (Monochrome Images ▸ Downsample to; doc_optimize mono); not separations, DeviceN, calibrated or ICC grey, or indexed images (their dark sample isn't read), images drawn or listed where /DefaultGray or /DefaultRGB is in force, images with a soft mask, halftones and dithers (most of their new pixels would cover both black and white, and their tones turn solid), images with a square of four such new pixels (a small screened picture on a white page; ragged scanned text has them along its strokes too, and keeps its resolution), or images wider than a fax row (1,048,576 samples), and Acrobat's bicubic and subsampling choices for monochrome aren't offered"
+evidence = ["crates/optimize/tests/optimize.rs::images_are_measured_where_drawn_and_downsampled", "crates/optimize/tests/optimize.rs::settings_choose_what_happens", "crates/automation/tests/it/automation.rs::optimizing_through_tools", "crates/optimize/tests/optimize.rs::a_page_picture_printed_as_one_pixel_strips_is_merged_into_one_image", "crates/optimize/tests/optimize.rs::images_that_do_not_tile_a_picture_are_left_alone", "crates/optimize/tests/optimize.rs::a_deeply_nested_form_is_measured_at_its_largest_use", "crates/optimize/tests/optimize.rs::images_are_not_resampled_when_some_uses_could_not_be_measured", "crates/optimize/tests/optimize.rs::image_resolution_uses_the_pages_physical_unit", "crates/optimize/tests/optimize.rs::optimization_retains_an_image_below_threshold_on_a_scaled_page", "crates/optimize/tests/optimize.rs::shared_forms_are_measured_in_each_pages_physical_units", "crates/optimize/tests/optimize.rs::ambiguous_monochrome_text_is_recompressed_without_lossy_reduction", "crates/optimize/tests/optimize.rs::indexed_thin_rules_survive_averaging_for_both_palette_orders_and_decodes", "crates/optimize/src/images.rs::an_indexed_images_palette_says_which_sample_is_dark", "crates/optimize/src/images.rs::it_matches_the_slow_way", "crates/automation/tests/it/automation.rs::doc_optimize_averages_black_and_white_scans_down", "crates/ui-egui/tests/it/create.rs::the_pdf_optimizer_dialog_saves_an_optimized_copy"]
+notes = "colour and grayscale, bicubic, effective resolution measured where drawn (every use, with page-local UserUnit scaling, through nested forms up to 12 deep; when a use can't be measured, nothing is resampled); image fragments stacked edge to edge are merged first (side-by-side tiles not yet); monochrome grey, image masks and indexed images (1-bit, CCITT fax, and eight-bit black-and-white grey and RGB packed to one bit) averaged by area to 300 ppi above 450 ppi by default, a tie to the dark sample (black in grey, painted in a mask, the darker palette entry over DeviceGray or DeviceRGB, after a decode array of two different numbers), then stored as Group 4 or 1-bit Flate (Monochrome Images ▸ Downsample to; doc_optimize mono); not separations or DeviceN (their tint transforms aren't read), calibrated or ICC grey, indexed images over another base, images drawn or listed where /DefaultGray or /DefaultRGB is in force, images with a soft mask, halftones and dithers (most of their new pixels would cover both black and white, and their tones turn solid), images with a square of four such new pixels (a small screened picture on a white page; ragged scanned text has them along its strokes too, and keeps its resolution), or images wider than a fax row (1,048,576 samples), and Acrobat's bicubic and subsampling choices for monochrome aren't offered"
 
 [[feature]]
 id = "optimize.recompress-images"