Add highlight and text colour marks, or a seam to register one

[virafb]

@emdash-cms/admin@0.33.0 registers a fixed inline mark set in src/components/PortableTextEditor.tsx: bold, italic, underline, strike, code, link, subscript, superscript. Highlight and text colour are absent, and @tiptap/starter-kit@3.29.2 does not bundle extension-highlight, -text-style or -color.

They can't be added downstream. Per #2430 there is no plugin seam to register a TipTap mark, and read-back conversion drops unknown marks. Note #2430 cites the core package's converters; @emdash-cms/admin has its own copy — convertPTMarks (line 1281) drops any bare unknown mark — so fixing core alone won't fix the admin editor.

Ask: add Highlight and TextStyle/Color to the built-in set, or ship the seam proposed in #1996/#144 and preserve registered custom marks on read-back.

[virafb]

A second case for the seam, from a different direction than the mark vocabulary
in the original report.

Building a plugin block catalog against 0.32.0, we converted a long document
from hand-written HTML into native prose plus typed plugin blocks: 31 source
blocks became 110, of which 72 are now ordinary editable prose.

One category of content could not make that crossing. Designed inline emphasis,
a coloured <em> used in 8 headings and 3 paragraphs, is a mark, so
portableTextBlocks does not reach it, and a bare decorator would be dropped on
the first save, as this issue and #2430 describe. Every block containing one
therefore stays an opaque HTML atom.

That is the part worth adding: the cost is not only that a custom mark fails to
survive. It is that every paragraph and heading containing one has to stay out
of the editor, along with all the ordinary prose in it. A mark is inline, so
one unsupported span keeps its whole block hand-written. In our document that is
eleven blocks of otherwise perfectly editable text.

Not asking to reopen #2430 — the split described there makes sense, and this is
the feature half. Recording it here because #2473 is where the seam is tracked.