Answer stability round 1: 0 🔴, 3 🟡; pin the blank line's spaces in an item's code block, drop the timing-only tests, and file tab-indented lines as item 120

docs/decisions.md

@@ -828,8 +828,9 @@ changed, and a before-and-after figure in its PR; the gate times nothing. Measur
 `adfDocumentFault`'s shape and depth walks stay two — the parting gives depth its own code — at
 52 ms for a 9 MB document the emit takes 314 ms over. The 500-level guard runs after the block walk,
 so it bounds nothing in it: `matchBlankLine` continues a blank line through the open list items in
-one step, and an item reads a line's indentation only up to its own width, which took 20000 nested
-items and as many blank lines from 10.7 s to 39 ms.
+one step, which took 20000 nested items and as many blank lines from 10.7 s to 39 ms; and an item
+reads a line's indentation only up to its own width, though a tab an item's width splits still
+costs the whole line at each level (`todo.md` item 120).
 
 ## Only the hard break holds a raw newline