Introduce docs-driven public surface generation (module_mode: :docs) with
mix snakebridge.docs.manifest for building from Sphinx objects.inv files.
Add new module discovery modes (:exports, :explicit) for finer control over
generated wrapper scope without walking large package trees.

Key changes:
- adapter_env option for passing environment variables to Python adapter
  with per-pool override support in multi-pool configurations
- class_method_scope and max_class_methods guardrails for inheritance-heavy
  classes that would otherwise generate thousands of method wrappers
- on_not_found option (:error or :stub) for missing symbol handling
- scan_extensions option for including .exs files in used-mode scanning
- mix snakebridge.plan for previewing docs manifest generation size
- Manifest fixes for class module normalization and *_stream resolution
- Update snakepit to 0.12.0