Add a nanopy command and make demos importable by name

[zamkorus]

Closes #19.

Why

Demos could only be run as scripts, and picking one of the MP4 export examples
meant uncommenting a call at the bottom of the file (#19).

What students get

A nanopy command, installed with the package:

nanopy                 # list every demo with a one-line description
nanopy dots            # run one demo
nanopy --version

NaNoPy is installed as a second name for the same command, and
python -m NaNoPy works when the executable is not on PATH.

Every demo is also an importable, documented function:

from NaNoPy.demos import dots

help(dots)   # explains what the demo shows
dots()       # runs it

from NaNoPy.demos import list_demos
list_demos()

Each demo file still runs standalone (python -m NaNoPy.demos.dots).

Changes

• NaNoPy/cli.py holds the CLI, declared under [project.scripts] as both
  nanopy and NaNoPy. python -m NaNoPy and python -m NaNoPy.demos
  delegate to it. Running it with no demo lists them and exits 0.
• Module and demo() docstrings on every demo, written for students; help(<demo>)
  is the intended way to find out what a demo does.
• NaNoPy/demos/__init__.py exports each demo under its own name, plus a DEMOS
  registry, list_demos(), and demo_summary().
• Demos that executed at import time (mac_arm_ball, multiple_ball,
  warning_deprecated, font_bug_on_graph_window) are wrapped in demo(), so
  importing NaNoPy.demos never opens a window.
• MP4 export examples are selected with argparse instead of uncommenting code
  (Having to uncomment is not elegant. Use argparse to handle this instead. #19), and exported as mp4_bouncing_ball, mp4_rotating_square,
  mp4_advanced_recording, mp4_with_audio.
• The collision __main__ benchmark became collision_benchmark(); the six
  variants are exported as collision_*.
• input.py's demo is exported as keyboard_input so it does not shadow the
  input builtin.
• The two demos that end without the window being closed now release their canvas,
  so demos can be called back to back in one session.

Tests

tests/test_demos_package.py covers: every demo is callable and documented, the
registry matches the exported names, importing every demo module creates no canvas
and does not initialize SDL, every demo file keeps its __main__ entry point, and
the CLI lists/selects/rejects demos. tests/test_mp4_export_examples.py covers the
MP4 example CLI.

91 tests pass; ruff check, ruff format --check, and pyright (strict) are clean.
Every demo was smoke-run headless with SDL_VIDEODRIVER=dummy, and the installed
nanopy command was verified end to end.

Note: [project.scripts] only takes effect after a reinstall, so existing
checkouts need uv pip install -e ".[interactive]" (or pip install -e .) once
before nanopy appears on PATH.