feat: enforce one HTTP route per handler

[earguinarena]

Summary

• Enforces at most one HTTP route per handler across GET, POST, PUT, DELETE, and ANY.
• Centralizes validation in the shared HTTP decorator factory.
• Reports the handler name and all conflicting method/path pairs.

Validation

• 258 tests passed.
• python -m compileall -q src tests passed.
• git diff --check passed.

Parent test commit: bebf183.