Finalize fixture impact validation and full helper-hoist closure

[doublemover]

Parent inventory: #8108
Blocked by: #8118, #8125, #8126, #8127, #8128, #8129, #8130

Objective

Perform final fixture/generated-artifact impact validation and end-to-end closure after all helper-hoisting implementation issues land.

Shared Module Specification

• Validate that no fixture contains copied helper implementations after migration.
• Validate fixture metadata still parses and preserves expected shape.
• Check JSON escaping/null-field/path-separator behavior in generated expected outputs.
• Regenerate fixture outputs only from durable source generators.
• Update Audit duplicated helpers and design shared tooling/probe modules #8108 with implementation status and any intentionally unmerged helper exceptions.

Migration Checklist

• Run all focused helper tests added by implementation issues.
• Run python -m py_compile for all changed Python modules/scripts.
• Run native build.
• Run npm run test:objc3c:runtime-acceptance.
• Run affected runnable object-model/e2e suites.
• Run affected build/publish/materialize scripts.
• Run git diff --check.

Performance And Reliability Requirements

• Keep helper code small, dependency-light, and fast enough for normal development feedback.
• Add focused tests for edge cases before broad migration.
• Preserve existing output schemas and public behavior unless an intentional bug fix is validated.
• Keep production helpers separated from test-only helpers.
• Do not use tmp/ as durable source-of-truth; generated temporary files are allowed only as transient command inputs.

Fast Tests

• Confirm helper tests run quickly enough for normal development feedback.
• Confirm Python helper imports do not noticeably slow short scripts.
• Confirm C++ probe compile time does not regress materially from oversized headers.
• Confirm JSON escaping and snapshot serialization avoid obvious repeated allocation hot spots.

End-To-End Completion Criteria

• All implementation issues are closed or have documented follow-up blockers.
• No new helper duplication was introduced while migrating.
• Production code does not depend on test-only helpers.
• tmp/ is not used as durable source-of-truth.