#4011 docs(mutation): state the file-name rule that marks test support code

Summary

Follow-up to #3994 review (comment). The isTestSupportModule comment said shared packages stay opaque. The rule reads only the file name, so a *-fixtures subpath that a workspace package exports is looked through as well. @agent-device/test-kit stays opaque only because tests reach it through its barrel. The comment now states that rule. Comment only; no behavior change.

Validation

• pnpm check:affected --run --head 58056ff752: passed, exactCommit=true.
• Not device-facing.

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