Feature: add signature exposure for static inference

[cvanelteren]

Ultraplot does a lot of runtime composition (lazy loading, dynamic dispatch, and shared doc merging), which makes static analysis difficult because analyzers see only source-level declarations, not the final runtime-expanded API. The lazy loader compounds this by delaying object/materialization of the public surface. On top of that, _obfuscate_parameters historically replaced visible signatures with compact (**kwargs) forms to keep docs manageable, which also removed concrete parameter metadata from inspect.signature, so tools like Pylance had less to work with for hovers/call tips.

So the concrete problem was a combo of:

1. runtime indirection (lazy loading),
2. runtime signature/docstring composition, and
3. signature obfuscation for docs at the callable object level.

This PR separates the two processes by keeping a copy for the docs themselves while using a process that static analysers like Pylance can work with.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ultraplot/tests/test_docstring_helpers.py	92.30%	3 Missing ⚠️
ultraplot/internals/docstring.py	92.30%	0 Missing and 1 partial ⚠️
ultraplot/tests/test_kwargs_helpers.py	87.50%	1 Missing ⚠️

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