Adding SdkSettings utility class

[birkholz-cubert]

Adds cuvis.SdkSettings, a utility for writing the SDK's cuvis.settings file from Python instead of maintaining it by hand.
Ported from an internal tool where the same pattern was already in use.

• Build settings in code: cuvis.SdkSettings(force_gpu_mode="cuda", processing_thread_count=8).
• Inspect and edit them like a dict - it is a MutableMapping, and xml_str shows the exact file that will be written.
• Load an existing configuration from a settings file or from the directory containing one; keyword arguments override what was loaded.
• Use it as a context manager to serialize it into a temporary directory, for pointing other consumers at the same configuration.
• cuvis.init() accepts an SdkSettings directly, so cuvis.init(cuvis.SdkSettings(...)) works.
• Values are stored as strings: bool becomes true/false, an Enum becomes its value, anything else goes through str(), and None drops the entry.
• No new dependency - loaded files are validated with xml.etree.ElementTree rather than an XSD validator, which would have meant adding lxml.
• 24 tests in tests/test_sdk_settings.py cover coercion, the mapping surface, loading from a file and a directory, round trips, the rejection cases and the context manager.

Documentation lives in the SdkSettings class docstring rather than in the README.

Known limitation, deliberately out of scope: setting ids are free-form, so a typo produces a valid file the SDK silently ignores.
No authoritative list of setting ids exists in this repository or in cuvis-il; sourcing one from the C SDK would be a separate change.