Pedagogy: side-by-side Hinton of A, B, A+B, A·B, Aᵀ in matrix-operations notebooks

Add a side-by-side **static Hinton** comparison of `A`, `B`, `A+B`, `A·B`, and `Aᵀ` to the matrix-operations notebooks (`40_linear_algebra_1/40_matrix_and_vector.ipynb` and `40_linear_algebra_1/45_matrix_and_vector_numpy.ipynb`).

## Scope

- One small example with `A` and `B` chosen to make structure visible (e.g., `A` triangular, `B` sparse, or one symmetric).
- Multi-panel figure: 5 Hinton subplots side by side.
- One sentence per panel describing what the structure shows.

## Why

Hinton makes operations **visibly structural**:
- `A+B` shows mass adding.
- `A·B` shows columns of `B` becoming linear combinations of columns of `A`.
- `Aᵀ` is a *flip across the diagonal* — under Hinton this is immediately readable, while under `matshow` it's just "different colors in different cells".

This is a different angle from #371 (which is the *intro* — heatmap vs Hinton on a single matrix); this issue is about **operations** as visual transformations.

Depends on / pairs with #371.