Maintenance: migrate pylab → matplotlib.pyplot + numpy across notebooks (33 files)

`pylab` was deprecated in matplotlib 2.x and is discouraged because `from pylab import *` pollutes the namespace — names like `array`, `plot`, `linspace` appear without indicating whether they came from `numpy` or `matplotlib.pyplot`.

## Migration target

Replace `pylab` imports with explicit:

```python
import numpy as np
import matplotlib.pyplot as plt
```

Then qualify bare names: `array(...)` → `np.array(...)`, `plot(...)` → `plt.plot(...)`, `linspace(...)` → `np.linspace(...)`, etc.

## Per-notebook checklist

### 00_introduction (1)
- [ ] `00_introduction/00_introduction.ipynb`

### 10_root_finding (6)
- [ ] `10_root_finding/10_sequential.ipynb`
- [ ] `10_root_finding/20_bisection.ipynb`
- [ ] `10_root_finding/30_newton_raphson.ipynb`
- [ ] `10_root_finding/40_examples.ipynb`
- [ ] `10_root_finding/42_examples_beams.ipynb`
- [ ] `10_root_finding/45_newton_raphson_complex.ipynb`

### 20_probability (2)
- [ ] `20_probability/15_random_variables.ipynb`
- [ ] `20_probability/20_statistics.ipynb`

### 28_interpolation (1)
- [ ] `28_interpolation/00_interpolation.ipynb`

### 30_num_int (6)
- [ ] `30_num_int/00_zeroth_order.ipynb`
- [ ] `30_num_int/10_first_order.ipynb`
- [ ] `30_num_int/20_second_order.ipynb`
- [ ] `30_num_int/25_convergence_order.ipynb`
- [ ] `30_num_int/30_T_section_centroid.ipynb`
- [ ] `30_num_int/40_circular_section_MOI.ipynb`

### 35_sympy (1)
- [ ] `35_sympy/20_Beam_Reaction_Force_General.ipynb`

### 40_linear_algebra_1 (6)
- [ ] `40_linear_algebra_1/10_vector_addition_subtraction.ipynb`
- [ ] `40_linear_algebra_1/15_vector_addition_subtraction_numpy.ipynb`
- [ ] `40_linear_algebra_1/20_vector_dot_cross_product.ipynb`
- [ ] `40_linear_algebra_1/30_3D_line_plane.ipynb`
- [ ] `40_linear_algebra_1/37_Gram_Schmidt.ipynb`
- [ ] `40_linear_algebra_1/40_matrix_and_vector.ipynb`

### 50_ode (6)
- [ ] `50_ode/10_Forward_Euler.ipynb`
- [ ] `50_ode/15_Forward_Euler_Higher_Order.ipynb`
- [ ] `50_ode/20_Heun_Method.ipynb`
- [ ] `50_ode/30_Runge_Kutta.ipynb`
- [ ] `50_ode/35_Runge_Kutta_Higher_Order.ipynb`
- [ ] `50_ode/45_Example_Cantilever.ipynb`

### 60_linear_algebra_2 (3)
- [ ] `60_linear_algebra_2/120_System_Linear_Eq_Four_Node_Truss.ipynb`
- [ ] `60_linear_algebra_2/125_System_Linear_Eq_Five_Node_Truss.ipynb`
- [ ] `60_linear_algebra_2/220_Google_PageRank.ipynb`

## Per-notebook acceptance

- No `pylab` references remain (`grep -L pylab` clean).
- Imports use `import numpy as np` and `import matplotlib.pyplot as plt`.
- Notebook executes top-to-bottom (`jupyter nbconvert --execute`).
- Output figures still render correctly (visual sanity check before/after).

## Workflow

- Push only to `origin/main` per repo convention (Colab badge URLs encode the branch name and don't round-trip from feature branches).
- Commit one folder at a time so the CI Clean-ipynb auto-commit stays scoped.
- 33 notebooks is large; expect to do this incrementally over several sessions.

Will be partly mooted by #388 once local pre-commit cleaning lands, but the pylab → pyplot/numpy migration is independent and worth doing on its own.