Add worked-example notebook (+ tolerant method-name lookup)

[tjayasinghe]

Summary

Adds a worked-example Jupyter notebook that teaches cuPeriod on real light curves,
and fixes a small documented-vs-actual method-name bug found while building it.

examples/cuperiod_tour.ipynb

A guided tour — for each object it shows raw light curve → periodogram → phase-folded:

Object	Method	Teaches
Classical Cepheid	GLS	the basics
RR Lyrae	MHAOV	sharp, multiharmonic pulsations
Eclipsing binary	BLS (+ the GLS P/2 trap)	choosing the right method
Long-period variable (Mira)	PDM	non-sinusoidal folds, long baselines
Exoplanet (Kepler KIC 7532973)	TLS	transit matched filter
—	several	comparing methods, reading N-best peaks

• Self-contained & offline: examples/data/ bundles 6 public ASAS-SN g-band light
  curves (one per variability class, with VSX periods) and one Kepler KOI (fetched once
  via lightkurve). No network needed.
• Outputs baked in, so the plots render directly on GitHub.
• Linked from the README and the docs quickstart. (examples/ is outside the sdist
  allowlist, so it doesn't bloat the PyPI package.)

Fix: hyphenated method names

The README/docs use "String-Length", but get_method did name.upper() →
"STRING-LENGTH" ≠ the registry key "STRINGLENGTH", so the documented spelling raised
UnknownMethodError. get_method, register, and MultiResult.__getitem__ now
normalize names (uppercase, strip non-alphanumerics): "String-Length" ==
"StringLength" == "STRINGLENGTH" (and "gls" == "GLS"). New registry tests cover it.

Verification

pytest 139 passed / 2 skipped, ruff + mypy clean, docs -W clean, notebook executes
end-to-end (0 errors, 6 figures).

🤖 Generated with Claude Code