35b - review: runs as a non-empty tuple, no item for a stray child reducing to nothing, 42 filed

todo.md

@@ -34,6 +34,10 @@
   paragraph of 2000 × `un` plus `**-r**`: each run its flanking cannot spell re-emits the whole line
   before riding the carry, quadratic in the runs (Goal 9), and the plain reduction's
   `spellableLine` drops one mark per re-emit the same way. Make both linear.
+- **42 — Trim a text leaf's trailing blanks in linear time.** `plain-inline.ts`'s `leafEdges` finds
+  the trail with an unanchored `/[ \t]*$/`, quadratic in a run of blanks inside one leaf: a
+  paragraph of `a`, 80 000 spaces, `b` takes 6.5 s in `adfToPlainMarkdown` (Goal 9). Scan backward,
+  as the expand title's trim does.
 - **34 — Read emphasis flanking by the whole character beside an astral symbol.** Check whether
   `line-escaping.ts`'s `charAt` and the parser's flanking read one UTF-16 unit beside an astral
   symbol — a lone surrogate is neither punctuation nor symbol, where CommonMark reads `😀` as