Tick 18, moving its text to todo-history.md

todo-history.md

@@ -820,6 +820,29 @@ The done `todo.md` items in full, as they were written. `todo.md` keeps a one-li
       dropping the outer link silently as `closeLink`'s `applyMark` does today, with a
       normalization fixture per shape (the stability-reviewer, 2026-09-16; the maintainer,
       2026-09-17).
+- [x] **18 — The subtree the directive spelling asks about (`0.2.0`).** The parser asks
+      `commonMarkSpelling` at every directive-spelled block and the answer emits the whole subtree
+      below, so a node at depth d is spelled d times: three nested rule-first directive lists cost
+      18 asks over 10 nodes, and 250 levels parse in 1.2 s at 16.4 kB, 4.9 s at 261 kB with a
+      kilobyte of content per level. The depth guard bounds the levels at about 250, never the
+      content, so this is the pipeline persona's hang on an input nobody typed (§11). Keeping each
+      child's emitted result for its parent's ask is not a straight handover: the same node object
+      is asked at different depths — 4, 3 and 2 for the innermost list of three — because the
+      parser counts a list and its item as two levels where the emitter's readable list counts one
+      (4b), and `headroom` is that guard's slack. The parts that survive the measurement: the paths
+      agree, `text` and `spelling` carry no depth, `headroom` is affine in it, and the parser asks
+      first at the deepest of them, so a kept result rebases by the difference. Either rebase and
+      record that argument in `AGENTS.md`, or give both directions one list accounting so a node
+      has one depth and nothing needs rebasing — which reopens 4b. A single post-build walk was
+      rejected: it reports the outer offender where the build reports the inner one (the
+      maintainer, 2026-09-18).
+      **Measured** (2026-09-19): the parse keeps each node's readable spelling (AGENTS.md §11), and
+      250 nested directive lists fall from 1.22 s to 0.07 s at 16.1 kB and from 5.20 s to 0.19 s at
+      261 kB; a panel between every pair of lists 1.00 s to 0.07 s, an opaque carry in every item
+      1.08 s to 0.04 s. The two depth-boundary tests are what pin the rebase — dropping it reddens
+      both — so no figure enters the gate (§14), as 4c settled for a behaviour-preserving cost fix.
+      The one list accounting was not taken: 4b settled that accounting the day this was filed, and
+      reopening it is an ask rather than a chunk.
 
 ## 5 — Ship `0.1.0`