M10: copy keeps formatting as HTML (Copy with formatting)

Copying selected page text now also puts an HTML fragment on the system
clipboard, so a paste into a word processor or mail client keeps
paragraphs, bold, italic, size, colour and font family. The plain text
is unchanged and stays on the clipboard as the alternative.

- export: clip.rs, the clipboard model (ClipText of paragraphs of
  styled spans), its builder with caps (20 000 spans, 2 MiB of text,
  16 MiB of HTML), font_family (subset tag and style suffixes off,
  camel case spaced) and clip_html. Family names are reduced to
  letters, digits, space, '-', '_' and '.', so a font name cannot leave
  the CSS string; text goes through esc, which drops control
  characters.
- engine: Document::rich_copy(page, text, glyphs). Glyphs (view space)
  take the style of the reading block their centre lies in; blocks are
  style-homogeneous. A line wrap inside a block is a space (none after
  a hyphen), a change of block a new paragraph. Refused when the
  document does not allow copying. compare.rs's display unit and
  weight/slant inference are shared, not duplicated.
- automation: text_extract takes format "text" (default) or "html";
  html adds each page's html and styles_incomplete, for the whole page
  or only what rect takes. Other values are InvalidArgs.
- ui-egui: Copy of a run or column selection writes HTML plus the
  plain text in one arboard set (desktop only); the web build and
  Select All of the whole document stay plain text. last_copied_html
  records what was produced, for tests.
- parity: view.copy-with-layout is partial; notes list what is missing
  (RTF, underline/strike-through, tables, Select All, web, glyphs
  outside reading blocks).

Formatting inference and the HTML serialiser are adapted from pdfcer
(MIT, same author) at 540b00ef1a405d1baa67c92f3858535037c7cde8.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SqmHYx8FiWM5PcsoiErgp9

parity/acrobat-features.toml

@@ -1571,11 +1571,11 @@ title = "Copy text keeping layout"
 area = "B"
 tier = "P2"
 milestone = "M3"
-status = "planned"
-commands = []
-tools = []
-evidence = []
-notes = ""
+status = "partial"
+commands = ["edit.copy"]
+tools = ["text_extract"]
+evidence = ["crates/export/src/clip.rs::heading_body_and_coloured_span_keep_their_formatting", "crates/export/src/clip.rs::special_characters_are_escaped_and_cannot_leave_the_style", "crates/export/src/clip.rs::empty_and_huge_selections_are_bounded", "crates/engine/src/rich_copy.rs::copy_keeps_headings_body_and_colour_as_html", "crates/engine/src/rich_copy.rs::copy_with_formatting_is_refused_when_copying_is_withheld", "crates/automation/tests/it/automation.rs::text_extract_as_html_keeps_the_formatting", "crates/ui-egui/tests/it/text_select.rs::selected_text_copy_keeps_the_formatting_as_html", "crates/ui-egui/tests/it/text_select.rs::selected_text_copy_with_formatting_honours_permissions"]
+notes = "Copy of a page-text selection also puts HTML on the system clipboard (paragraphs, bold, italic, size, colour, font family; plain text as the alternative), and text_extract format html returns the same. Missing: RTF, underline and strike-through, tables, Select All of the whole document (plain text only), the web build (plain text only), and styles for glyphs outside the page's reading blocks (copied unstyled)"
 
 [[feature]]
 id = "view.snapshot"