spec(maxpool2d_layer): a lowerable out_side(kernel, stride) for the nn-maxpool widget #10664

Why

gHashTag/trinity#1696 (course 10, neural networks) was refused for one defect (review 5565039954): the widget nn-maxpool hand-holds K_OUT_SIDES = [2, 2, 3, 1]. Those are out_size(4, cfg(k, s)) from specs/ml/layers/maxpool2d_layer.t27, but out_size takes a MaxPool2DConfig struct and does not lower to WASM, so logic.js lists it in __NOT_LOWERED__ and the widget has no compiled way to get the output side.

What

Add a lowerable fn on two integers:

fn out_side(kernel: u32, stride: u32) -> u32 {
    return out_size(4, cfg(kernel, stride));
}

plus tests:

• the widget's four configs (K_KERNELS = [2, 3, 2, 4], K_STRIDES = [2, 1, 1, 1] on trinity's feat/course-neural-networks): (2,2)->2, (3,1)->2, (2,1)->3, (4,1)->1;
• edges: a 1 x 1 window keeps the side (1,1)->4; a stride that does not divide rounds down, (3,2)->1 and (1,3)->2.

Done when t27c test-report is all PASS with 0 vacuous, a mutant of the fn goes red, the seal is saved, and t27-logic.mjs lists outSide among the lowered fns.

Not here: re-vendoring into trinity, calling outSide(k, st) in tool.js and dropping K_OUT_SIDES (a later step on trinity#1696).

Boundary

• specs/ml/layers/maxpool2d_layer.t27
• .trinity/seals/Maxpool2d.json, .trinity/seals/layers_Maxpool2d.json (its seal files)
• docs/now/2026-10-18-maxpool-out-side.md (NOW entry)

Parent: #9063