Preserve documented external core helper APIs

[cofin]

Summary

Documented or externally callable core helper surfaces should remain available even when they are not called internally by SQLSpec.

Scope

• Keep documented helper exports additive across sqlspec.core, cache, hashing, and pipeline modules.
• Preserve external cache deletion helpers, filter-view methods, and result helper APIs.
• Add smoke coverage so future internal cleanup does not remove externally callable surfaces by accident.
• Fold only private helpers that have no public or documented call surface.

Validation

Covered by cache, result, parameter, and core unit tests in PR #545.