docs: define typed user config boundary

[codeforester]

Summary

Refs #105.

• document the intentional opaque Context.user_config: object | None boundary
• recommend one consumer-owned typed accessor instead of repeated casts
• define why a fourth Context type parameter is deferred beyond 0.4.x
• specify the end-to-end design and migration requirements for a possible 0.5.0 generic expansion
• link the proposal from the consumer-profile docs and README

Validation

• bash tests/validate.sh
• python scripts/validate_docs.py
• python -m pytest -q
• ruff check lib/python scripts examples tests compatibility

[codeforester]

Superseded: issue #105 is categorized as enhancement, so the branch must use the canonical enhancement/105-YYYYMMDD-* pattern. A replacement PR will be opened from the corrected branch.