[bobsingor]

/RC is an entry of every markup annotation (ISO 32000-2 §12.5.6.2, Table 171): the formatted text its popup shows. Only a free text draws it. Until now our rich text setters only wrote a free text, so a note's comment couldn't be written formatted, and an edited note kept Acrobat's old /RC next to its new /Contents.

• EPDFAnnot_SetRichTextJSON and EPDFAnnot_SetRichTextXHTML now write any other markup annotation's comment (text, line, square, circle, polygon, polyline, the four text markups, caret, stamp, ink, file attachment, sound, redact, projection): /RC and its plain projection in /Contents, nothing else. No /DS, /DA or appearance, as Acrobat writes a note's.
• The body states only what differs from a comment that states nothing (the reader's face, 12 pt, black, left-aligned). A comment written with the body it was read with comes back the same, and Acrobat's style-less notes stay style-less.
• Links, popups and widgets are refused and left unchanged.

The writer (CPDF_RichTextWriter::ApplyComment, SerializeCommentRC) shares the envelope with the free text one; SerializeRC is unchanged.

Checked against Acrobat

text_rich_comment_acrobat.pdf is a note written by Acrobat 26: no body style, Enter as 
 inside a span, and font-size:-14.0pt on every span, a size nothing can be. We read it as rich text, the body's size kept, and writing it back reads the same, without the impossible size.

Tests

• AcrobatNoteCommentReadsAsRichText: Acrobat's note reads as rc with its bold, underline and italic runs and its line breaks.
• SetRichTextJSONWritesACommentAsAcrobatDoes: a note's exact /RC (no body style) and /Contents, no /DS, /DA or /AP; a square whose body differs states only its size and colour.
• AcrobatCommentWrittenBackReadsTheSame: Acrobat's comment, read and written back, reads the same.
• SetRichTextJSONRefusesKindsThatArentMarkup: a link and a popup are refused.
• The two writing tests are recorded as layer-mode writers.

Three deliberate breaks in the new code each failed a test. On macOS the 203 annotation and rich text embedder tests pass. The engine's annotation conformance passes on the local engine (1923) and the cloud engine (1120) against a runtime built from this branch. The full suites and Linux CI haven't run yet.

Found on the way, not changed here: inside a layer document, writing to this fixture's note promotes no object into the layer. Upstream's FPDFAnnot_SetStringValue does the same, so it isn't new. The file keeps its objects in object streams, which may be why.

The engine side (richText on every markup kind, and a contents edit rewriting a formatted comment instead of leaving stale /RC) is on embedpdf/embed-pdf-viewer feature/markup-rich-text, which needs this released and pinned.