feat(multi-runner): add experimental config translation

[edersonbrilhante]

Description

• Add a typed experimental configuration contract to the multi-runner module.
• Normalize stable inputs and experimental global/lane values into one canonical shape.
• Preserve the existing stable resource and module routing; this change does not call runner-config, orchestration-provider, or compute-provider modules and does not change Terraform resource addresses.
• Add focused selector and precedence tests, generated input documentation, and multi-runner Terraform test coverage.

This is the contract-and-translation slice split from #5251. Runtime provider wiring and address documentation are intentionally deferred to the follow-up PR.

Test Plan

• tofu test in modules/multi-runner: 3 passed, 0 failed
• terraform validate with Terraform 1.3.10 and 1.5.6
• Terraform test coverage with Terraform 1.14.9
• pre-commit Terraform formatting, TFLint, and merge-conflict checks
• Generated README verified against the repository workflow's terraform-docs v0.20.0 output

Related Issues

• Related to feat(multi-runner): add experimental EC2 provider stack #5251
• Related to refactor(multi-runner): decouple EC2 Terraform logic into provider modules #5252

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None