feat: publish stable typed extension contracts (#59)

[codeforester]

Summary

Implements #59 by publishing a stable, typed extension boundary for professional Python CLI consumers.

What changed

• Added public RuntimeLayout, profile resolver/history protocols, and generic Context[ConfigT, ApplicationStateT, ServicesT] contracts.
• Added ParamSpec-preserving command/subcommand/decorator APIs and concrete Click subtype-preserving attach() return types.
• Added typed AttachmentAdapter/AttachmentContract boundaries for attached Click trees.
• Replaced command-protocol-only process-global usage with instantiable CommandSchemaRegistry and CommandCodec, while preserving module-level compatibility aliases/helpers.
• Made native async policy explicit: async callbacks and awaitable callback results are rejected deterministically, with cleanup-safe handling.
• Added identity-safe attachment sentinels and transactional collision rejection for reserved Click markers.
• Added strict typed consumer example, docs, changelog, CI mypy gate, and regression coverage.

Validation

• python -m pytest -q — 324 passed
• mypy --strict examples/typed_consumer.py — passed
• python -m build --outdir /private/tmp/base-cli-59-dist — sdist and wheel built successfully
• git diff --check — passed

Closes #59