[codex] Consolidate statement phase dispatch

[jsiek]

Summary

• add a shared statement-kind registry for the proof-checking lifecycle phases
• route process_declaration_visibility, process_declaration, type_check_stmt, collect_env, and check_proofs through common dispatch helpers
• add unit coverage that the statement phases share the same registry

Validation

• python3 -m py_compile proof_checker.py
• pytest test/unit
• python3 test-deduce.py

Codex session

Raw URI: codex://threads/019e2580-8773-76c0-99bb-44f4a1d0eaa6

open 'codex://threads/019e2580-8773-76c0-99bb-44f4a1d0eaa6'

[jsiek]

I'm not so sure I like this refactoring. It increases code size with a marginal benefit.

[jsiek]

Instead of a dispatch table I'm leaning towards doing the object-oriented thing and making them methods of the AST classes.