refactor: decompose modules by vocabulary

[binaryaaron]

Summary

• split display rendering, span handling, and payload coercion into named modules
• separate NDD observability from adapter execution and facade support from the public interface
• add import-linter boundary contracts and the import-linter development dependency

Verification

• make check
• make test
• import-linter contracts
• symbol-location and structural gates from the migration plan
• final vocabulary legibility probe

Follow-up

• stacked measurement tooling refactor: refactor(measurement): decompose tooling by vocabulary #226