uselesskey-core-negative-der

DER-specific negative fixture builders for deterministic corruption used by uselesskey test utilities.

Purpose

• Truncate DER vectors and flip bytes at deterministic offsets.
• Provide deterministic DER mutation helpers used for parser-failure fixtures.
• Keep DER behavior isolated as a small SRP microcrate.

This crate intentionally only manipulates byte shape and does not parse or validate cryptographic semantics.