test(scripts): floor partition-test-shards' self-test via the thunk route (#13799 batch 4) (#15198)

Its 36 assertions are bare `throw`s, which is why batches 2 and 3 deferred
this file twice: routing them through a boolean helper means inverting 36
failure conditions by hand, and a dropped `!` yields an assertion that still
registers its case and still passes -- invisible to the very floor being
installed.

The thunk route touches no condition. `check(() => { ... })` counts a case and
then runs the existing `if (...) throw ...` verbatim, carried in modulo
indentation only; 22 sites were single-line and 14 multi-line, several nested
in a `try`/`finally` and behind a dataset-presence guard. Seven batteries are
declared from the seven banners that register cases (2/1/1/2/9/5/16 = 36), the
roster's own size is pinned, and the floor is evaluated before the verdict so
the success line cannot be printed by a run whose batteries went quiet.

The self-test's printed verdict is byte-identical to the base tree's.


Claude-Session: https://claude.ai/code/session_012zGPuVVX3deAx9LdjK8jCk

Co-authored-by: Claude <noreply@anthropic.com>

Author: claude[bot]