Publish stable typed runtime, profile, and context extension contracts

[codeforester]

Parent: #50

Train position: 9 of 22
Phase: P1 — Incremental adoption

Problem

Public extension types depend on private implementation classes, and decorators, callbacks, context payloads, and Click objects are weakly typed.

Acceptance criteria

• Public profile/runtime protocols or dataclasses no longer require importing private RuntimeLayout.
• Resolver, history/finalizer, runtime binding, and Context extension points are documented public contracts.
• Decorators preserve call signatures with ParamSpec, callbacks and Click objects are typed, and Context supports generic consumer config/state payloads.
• Global mutable command-schema state is replaced by an instantiable registry/codec contract or isolated behind a compatible public object.
• Native async callback support has an explicit documented decision and, if supported, a tested lifecycle contract.
• A strict-typechecked sample consumer passes and compatibility for current CliProfile/RuntimeBinding is covered.

Dependencies

• Make lifecycle teardown failure-safe #51
• Repair application naming and registration contracts #55

[codeforester]

Carryover from #57/#79 review: preserve concrete Click command subtypes with a generic attach return TypeVar; make application context and services payloads generic; extract the roughly 1.2k-line private attachment adapter from app.py behind a typed internal contract; and replace truthy reserved instance instrumentation markers with identity-safe sentinels or a registry plus transactional collision rejection.