Improve plotting utilities and robustness

[nirum]

Summary

• harden palette plotting, respect confusion-matrix opts, and keep generator inputs working across plot helpers
• scope image colorbars to their axes, safeguard signal normalization/stable rank, and flush stopwatch timing output
• document updated behaviours, satisfy pyrefly typing, and broaden tests (including ridgeline color handling and ellipse coverage)

Testing

• ============================= test session starts ==============================
  platform darwin -- Python 3.12.9, pytest-8.3.5, pluggy-1.6.0
  rootdir: /Users/niru/code/jetplot
  configfile: pyproject.toml
  plugins: cov-6.1.1
  collected 40 items

tests/test_colors.py ... [ 7%]
tests/test_images.py ..... [ 20%]
tests/test_plots.py ............. [ 52%]
tests/test_signals.py ....... [ 70%]
tests/test_style.py ... [ 77%]
tests/test_timepiece.py .. [ 82%]
tests/test_utils.py ....... [100%]

=============================== warnings summary ===============================
tests/test_signals.py::test_stable_rank
/Users/niru/code/jetplot/tests/test_signals.py:17: RuntimeWarning: divide by zero encountered in matmul
X = U @ np.diag(S) @ V.T

tests/test_signals.py::test_stable_rank
/Users/niru/code/jetplot/tests/test_signals.py:17: RuntimeWarning: overflow encountered in matmul
X = U @ np.diag(S) @ V.T

tests/test_signals.py::test_stable_rank
/Users/niru/code/jetplot/tests/test_signals.py:17: RuntimeWarning: invalid value encountered in matmul
X = U @ np.diag(S) @ V.T

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
================================ tests coverage ================================
_______________ coverage: platform darwin, python 3.12.9-final-0 _______________

Name Stmts Miss Cover

src/jetplot/init.py 9 0 100%
src/jetplot/chart_utils.py 130 6 95%
src/jetplot/colors.py 68 10 85%
src/jetplot/images.py 85 18 79%
src/jetplot/plots.py 167 9 95%
src/jetplot/signals.py 38 2 95%
src/jetplot/style.py 39 12 69%
src/jetplot/timepiece.py 67 12 82%

TOTAL 603 69 89%
======================== 40 passed, 3 warnings in 3.17s ========================

• All checks passed!