CI blind spot: interactive ipywidgets callbacks aren't exercised (slider-state bugs slip through)

## Problem
CI runs notebooks via `jupyter nbconvert --execute` — each cell once, top-to-bottom, with **no widget interaction**. So bugs that only appear when a user *interacts* with an `ipywidgets` control (moves a slider), or that depend on **shared global state mutated by later cells**, are invisible to CI.

## Concrete example (just fixed in `60_linear_algebra_2/200`)
The Power Method arrow-slider callback `show_iterate(step)` read the global `n`. Cells 49/52/58 (`lam, vecX, n = power_method(...)`) overwrite `n` → `99999` for the non-converging "What if" matrices. After running those cells, **moving the slider** raised:

```
ValueError: shape mismatch ... (99999,) vs (2,) # ax.bar(range(99999), vec[2])
```

CI passed cleanly because:
- the `interact` callback only fires once, at creation, when `n` was still correct;
- nbconvert never moves the slider, and never re-fires the callback after `n` is clobbered;
- the `if os.getenv('CI'): show_iterate(last)` guard renders one static frame — by design, to keep CI green — which also means the interactive path is never exercised.

(Fixed by capturing the dimension locally: `dim = vec_array.shape[1]`, independent of the mutable global `n`.)

## Options
1. **Convention (cheapest):** widget callbacks must capture all needed state **locally** in their own cell, never reading mutable globals that later cells reassign. Make it a review/lint checklist item.
2. **Exercise callbacks in a test:** for notebooks containing `interact(...)`, after executing all cells, call the callback across its full slider range and assert no exception. Non-trivial to wire generically.
3. **Static check:** flag a widget callback that references a module-level name reassigned elsewhere in the same notebook. Hard to do reliably (needs data-flow analysis).

## Related
Same family as #440 and the "missing Colab clone cell" gap — CI executes the *repo, non-interactive* path, not the *interactive/Colab* path.

🤖 Filed via Claude Code after fixing the 200 slider crash.

Start by reviewing the CI command, the interactive notebook path 60_linear_algebra_2/200, and the related issue #440. Decide which of the three proposed approaches is in scope, then define a reproducible check for callbacks whose state changes after later cells run. Done should include coverage for the slider-state failure without breaking the existing non-interactive CI path.

Written by the indexing model from the issue text.