fix: repair application registration contracts

[codeforester]

Summary

• make App.name authoritative across Click materialization, direct invocation, run_app, help, version, and runtime identity
• freeze registration transactionally and reject duplicates, late mutation, reentrant builds, and custom command-class name overrides
• make module-level command decorators independently retrievable and runnable with stable cross-Click name inference
• document the registration lifecycle and exported get_command_app contract

Validation

• full 244-test suite on Click 8.4.2
• full 244-test suite on Click 8.1.8
• 147 downstream Base engine tests plus 10 subtests
• isolated wheel and sdist build and artifact validation
• adversarial concurrency, naming, decorator, and public API review

Closes #55