Maintenance: remove scratch files from repo and tighten .gitignore

A handful of scratch / experimental files have accumulated at various points in the tree (some tracked in history, some currently in working copies). Clean them up and tighten `.gitignore` so they stop reappearing.

## Files to remove if currently tracked

(Verify each with `git ls-files ` before `git rm` — some may be untracked working-copy artifacts only.)

- `Untitled.ipynb`
- `35_sympy/del_this.ipynb`
- `00_introduction/a.out`, `00_introduction/hello.c`, `00_introduction/hello.txt`
- `60_Duffing_Oscillator.bak.ipynb` (or wherever the `.bak` lives)
- `50_ode/ODE examples *.ipynb`
- `pip_list.txt`, `pip_list_github.txt`
- Any other `.bak.ipynb` or `Untitled*.ipynb`

## .gitignore additions

- `.DS_Store`
- `*.bak.ipynb`
- `Untitled*.ipynb`
- `pip_list*.txt`
- `a.out`
- Any compiled-output patterns from the C++ markdown-cell tests (objects, executables under module dirs).

## Why

These files clutter `git status` (and PR diffs) for everyone, get accidentally added to commits, and offer no value to learners. A one-time cleanup plus a stricter ignore list pays back permanently.

Mechanical, low-risk, easy to review.