span-marks identifies a hard break by comparing an atom's text to the printed image " +"

Split from #325, row R3 (verified at main ffdc1860).

The question: is this atom a hard break?

The reader's answer: the node kind HardLineBreakNode (src/ast.ts), built in inline-link-builder.ts.

The second answerer: src/print/span-marks.ts l.78, inner.at(-1)?.text === HARD_BREAK_IMAGE, a comparison against the literal " +". Any other construct whose atom text is " +" answers the same. The one place that knows the atom is a break is where it is built in src/print/inline.ts, and nothing marks it there.

Closes when: the atom carries the kind it was built from (or the check reads the node) and the image comparison is deleted. Red-first pin: a text atom whose bytes are " +" but which is not a hard break (an escaped or passthrough plus at line end) beside a span mark.

Related: #325 (the catalog), #90 (the class), #323 (trailing + and {plus} spellings).

Start with the HardLineBreakNode definition in src/ast.ts, its construction in inline-link-builder.ts and src/print/inline.ts, then inspect the comparison at src/print/span-marks.ts line 78. Add a red-first case where a non-break text atom has bytes " +" at line end beside a span mark. Done means the atom carries or exposes its node kind and span-marks no longer relies on the literal image comparison.