fix(core): expose lazy imports to introspection

[cupkk]

Summary

Make the lazy genblaze_core public surface discoverable through dir() and export RunnableConfig from the top-level package.

Changes

• add RunnableConfig to the core lazy import table
• implement genblaze_core.__dir__() using the umbrella package pattern
• cover pre-access discovery and top-level import identity
• document the import surface and add an unreleased changelog entry

Test plan

• make test
• ruff check libs/ cli/ examples/
• ruff format --check for the changed Python files
• make deptry
• make lint (the format step reports the same 17 pre-existing README code-block formatting issues on clean ec81e81 with ruff 0.16.0)
• Docs updated

Related

Closes #55

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