Reproducible-build release gate (StageX)

[abienkowski]

Problem

The repo uses StageX source-bootstrapped base images for reproducible Docker builds, but reproducibility is not enforced as a release precondition.

Solution

1. Run the reproducibility verification targets for all three languages:
   • make verify-reproducible-go
   • make verify-reproducible-rs
   • make verify-reproducible-ts
2. Decide whether reproducible builds should be a hard release gate (already a CI job) and document the expected hashes
3. Verify the docs at docs/reproducible-builds.md match current behavior

Which implementation(s) would this affect?

• All

Additional context

• Makefile targets: verify-reproducible-*
• CI: reproducible-build job in .github/workflows/ci.yml
• Reference: docs/reproducible-builds.md, https://stagex.tools/
• Tracked by epic First release prep: Quint spec review, test parity, and release pipeline validation #4