refactor: graduate DefaultOptionKeys out of experimental namespace

[TKaltofen]

Summary

• Moves DefaultOptionKeys from mloda_plugins.feature_group.experimental.default_options_key to mloda_plugins.feature_group.default_options_key as a stable import path
• Adds a deprecation re-export shim at the old path that emits a DeprecationWarning pointing to the new location
• Updates all 77 internal imports across core, plugins, tests, and docs to use the new path
• Adds graduation tests verifying the new path works, the old path re-exports the same class, and the deprecation warning fires

Closes #233

Test plan

• All 5 graduation tests pass (new path import, key existence, value correctness, re-export identity, deprecation warning)
• All 6 existing test_default_options_key.py tests pass via the shim
• Full tox run: 2196 passed, 124 skipped (13 pre-existing failures unrelated to this change)
• ruff check clean on all changed files
• mypy --strict clean on all changed files