Design-first plan, ruling before any build (same gate as #76). Band =
foundational spine core: seed-simple + comparison + question-arithmetic
(simple/summation/difference), on the shared #78 polarity substrate.
Bakes in Josh's four refinements: (1) deliberate ~30 smaller-band rationale
(first end-to-end reader conversion ever + two reusable foundations, zero new
hard capabilities beyond #78); (2) done-when reset to band-solve>0 wrong=0 +
foundations validated, ~40 bar cumulative across increments 1-3; (3) haircut
factor (real/upper-bound) as a named output; (4) TWO wrong=0 direction drivers
— #78 statement polarity AND q:difference question direction, both positively
determined + fail-closed. Grounds design on what EXISTS (question layer partly
built: summation works, q:difference partial). Schedules q:complex decomposition
next (may be corridor-tractable, could move the ~48% ceiling).
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