Coarse-graining support: Support.CoarseGraining.BlockResponse.Perturbation

Imported from the Apache-2.0 CoarseGraining development at commit c7ddd76c08ade64fed1b8d2ca51be14dfee8deb4.

BlockResponse perturbation, first-variation, and witness identities

(aggregate re-export)

Previously a 2169-line monolithic module; now split along thematic boundaries into the five files imported above. Shim for backward compatibility.