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

Why

Several modules had accumulated responsibilities with different reasons to change. Display rendering shared a module with span handling and payload coercion, NDD execution shared one with observability, and the public facade also owned output and telemetry support. This made ownership unclear, increased coupling, and made structural regressions hard to detect. The refactor gives each concern a named module and adds enforced import boundaries while preserving existing behavior.

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