Stage 21 amendment: unsolvable-cell rate, shortcut census, undecided label

Registered after the asis rep-1 arms started but before any aggregate
score was read (disclosed in the doc). Prompted by an agentic-benchmark
audit; three reporting additions, no change to arms, prompt, sample,
model or scorer.

A cell counts as solved at f1_best >= 0.5. Cells no arm ever solves are
counted and printed as their own number, and every paired difference is
reported twice: over all sampled cells, and over the subset at least
one arm solved. Zero-in-both cells pull a paired difference toward zero
and would otherwise read as arm equality when they are signal absence.

The read-it-whole shortcut is measured, and it is large and confounded
with source: against library_read's 24,000-char default and the 64,000
tool-result ceiling, 39.4% of the n=160 sample fits one default read
and another 28.1% fits one deliberate read, so 67.5% of cells have a
whole-document shortcut available. contractnli is 38/40 in the smallest
band; maud is 40/40 above 200k. Paired differences are therefore
reported stratified by document-size band, with the shortcut-TAKEN rate
(one read covering >=95% of the document) beside it.

Differences smaller than the measured within-arm replicate floor are
labelled undecided rather than null.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H9ToHYJVDxfeJcJwdzrP2H