Pedagogy: unify 60_linear_algebra_2 elimination sliders (shared hinton_step_slider; add to 100 + 125)

[kwlee2025cpp]

Context

The 60_linear_algebra_2 chapter now has Hinton step-sliders in several elimination / iteration notebooks, but they are built two different ways:

• 150 (Inverse matrix / Gauss–Jordan) uses the shared helper matshow.hinton_step_slider, which computes one common_max_weight across all frames (box sizes are comparable step-to-step — you can watch a pivot shrink), is CI-guarded (renders the final frame headless, interactive in Colab), and accepts a titles= argument. Used at 3×3 / 4×4 / 7×7.
• 102 (Gauss elimination, n×n) has a bespoke inline slider (gauss_elimination_steps(...) → ipywidgets.interact/IntSlider) with nice per-step titles, but each frame calls a local hinton(matrix) with no fixed scale, so every frame rescales independently — a box of a given size can mean different magnitudes from step to step.

Proposed

1. Unify 102 onto matshow.hinton_step_slider. Keep the per-step titles (the helper takes titles=), gain the shared scale (cross-step comparability — the whole point of the slider), and drop the duplicated bespoke code + the local hinton definition.

   • 102 currently defines its own local hinton; switching to the helper means import matshow, so confirm 102 has the Colab clone cell that clones kwlee2025cpp/nmisp_py (cf. CI can't catch a missing Colab clone cell — add a static check (tests/test_colab_clone_cell.py) #442). 150 already has it.

2. Add the slider to notebooks that lack one:

   • 100 (Systems of Linear Equations) — has hinton() but no slider.
   • 125 (Five-Node Truss) — no Hinton / slider at all.
   • Both walk through an elimination/solve that a per-step scrubber would make clearer.

Done =

• Where a gauss_* helper supports it, collect frames via a snapshots= list (as in 150); otherwise retain per-step matrices in the existing loop.
• Drive matshow.hinton_step_slider(snapshots, titles=...); keep the CI guard (already in the helper).
• Verify headless (CI=true jupyter nbconvert --execute); introduce no new Colab-clone gaps (CI can't catch a missing Colab clone cell — add a static check (tests/test_colab_clone_cell.py) #442).
• Concept-name cross-references only (renumber-proof).

Parked from the 2026-06-01 session (sliders were just added to 150 4×4/7×7 in #444); building deferred.