M11: an indexed image's palette is read once it matters, within 4 KiB; a colour key keeps its image

From a review of the spellings an indexed image can take:
- The palette was read for every 1-bit indexed image, resampled or not, and twice each. Images
  share a table, so 1,000 images sharing one with 16 MiB of raw data took 16 to 60 s, none of
  them resampled. It is now read only for an image about to be resampled, and a table over
  4 KiB is refused before it is copied or decoded, since no palette needs that.
- A colour key can hide the dark sample a tie goes to, so a thin rule of the other was lost: a
  white rule on a hidden black went from 255 to the backdrop's 128. An image with a colour key
  now keeps its resolution. Grey had this since #5426 and is covered too.
- `/Indexed` by reference, and a base of `[/DeviceRGB]`, are the same colour space and are read
  so. `/I`, an inline abbreviation, and a real hival aren't, and such an image is left alone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AmsdeUCdTaD72jkDw2pJKY

parity/acrobat-features.toml

@@ -7385,7 +7385,7 @@ 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/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"
+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 or with a table over 4 KiB, images drawn or listed where /DefaultGray or /DefaultRGB is or may be in force (an indexed image also used beyond the page and form resources read, as an annotation's appearance), images with a colour key, which can hide the dark sample, or 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"