12c: the block walk matches only list items and blockquotes per line and finds a closer's container by name, replacing the depth guard

todo.md

@@ -74,9 +74,12 @@ proves 12, 13 spells 11's gaps in 12's grammar, and 12 rewrites code 4b and 4c c
       them: `readNestedDirective` restarts its depth counter per level, so each parse level
       re-scans the region below it and nested inline directives cost O(depth × content) — 3f's
       cost, which 3i's slot parse doubles rather than changes in class, bounded by the 500-level
-      guard. §11's scanning rule is the whole argument; the pipeline persona feeds documents
-      nobody typed. `readDirectiveContent`'s scan splits into named steps with that fix rather
-      than keeping its complexity (the maintainer, 2026-09-16).
+      guard. A fifth predates 12c: a blank line continues every open list item without consuming
+      input, so the block walk visits the whole item stack per line — `d` nested items and `4d`
+      blank lines take 11s at 59 KB (the stability-reviewer, 2026-09-16). §11's scanning rule is
+      the whole argument; the pipeline persona feeds documents nobody typed.
+      `readDirectiveContent`'s scan splits into named steps with that fix rather than keeping its
+      complexity (the maintainer, 2026-09-16).
 - [ ] **4d — What the gate says while it runs (`0.2.1`).** `ci.sh` runs nine legs and announces
       none of them, so five minutes of a Gitea run read as silence and a hang cannot be told from
       a slow pull — the maintainer hit exactly this on the `0.1.0` release. Three causes, each its