[Epic] Thorough codebase validation

[djm81]

Why

Parent issue for thorough codebase validation (repro, contract-test, sidecar, dogfooding). Use this issue to group and track validation-depth work.

What

• Area: Validation (specfact repro, contract-test-full, deep CrossHair/Semgrep)
• Purpose: Assign child issues and change proposals to this parent via Relationships or project Type grouping.
• OpenSpec: Changes under openspec/changes/validation-01-*. See openspec/CHANGE_ORDER.md.

Acceptance Criteria

• All validation-related issues are linked to this parent.
• Implementation order follows CHANGE_ORDER.md.