TypeScript guidelines: Document any requirement for callback parameters subject to bidirectional assignment

[MajorLift]

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I keep encountering this pattern. Seems worth documenting, especially since function contravariance is confusing enough on its own.

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Note

Low Risk
Documentation-only change with no runtime behavior impact; risk is limited to potential confusion if the guidance is misapplied.

Overview
Adds a new TypeScript guideline section explaining when any is required for callback parameter types that must be assignable in both directions under --strictFunctionTypes (bidirectional assignment).

Includes an illustrative Wide/Narrow example, rationale for why unknown/never fail, guidance on scoping any to parameter positions (with intentional ESLint disable), and a couple of real-world references (e.g., messenger handler slots, coerces maps).

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