Add a guardrail test to prevent future reintroduction of Bazel legacy feature dependencies

[nradakovic]

What

Add an automated guardrail that detects if the toolchain starts depending on Bazel legacy C++ feature behavior again in the future.

The goal is to protect the explicit-feature migration from regression after the initial implementation is complete.

Acceptance Criteria (DoD)

• An automated check exists that fails if the toolchain regresses back to relying on Bazel legacy feature defaults.
• The check runs as part of normal validation or CI.
• The guardrail is deterministic and maintainable.
• The failure mode is understandable enough that maintainers can act on it quickly.

How

• Choose a guardrail mechanism appropriate for this repository, such as:
  • a dedicated validation target
  • an analysis-time assertion
  • a golden feature-list check
  • another deterministic check tied to the explicit feature model
• Integrate the guardrail into CI so it runs continuously.
• Ensure the check is tied to the explicit-feature contract established by the migration.
• Document what the guardrail validates and how to interpret failures.

Out of Scope

• Replacing the broader functional test matrix.
• Adding unrelated policy checks to CI.
• Redesigning the repository's entire validation strategy.