Motivation

• Multiple keypair crates (RSA, ECDSA, Ed25519) implemented identical PKCS#8/SPKI accessor, tempfile writer, negative-fixture, mismatch, and kid methods, creating duplication and maintenance burden.
• Centralizing these accessors reduces repetition and ensures consistent behavior across key types.

Description

• Add a shared, exported macro impl_pkcs8_spki_fixture_accessors!() in crates/uselesskey-core/src/srp/keypair_material.rs that generates the common accessors, tempfile writers, corruption helpers, mismatched-public-helper, and the kid accessor.
• Replace the repeated inherent method implementations in crates/uselesskey-rsa/src/keypair.rs, crates/uselesskey-ecdsa/src/keypair.rs, and crates/uselesskey-ed25519/src/keypair.rs with a single invocation of the new macro.
• Clean up imports in the affected keypair modules to reflect the macro-driven implementation and reduce unused use lines.
• Keep the existing Pkcs8SpkiKeyMaterial API intact and reuse its methods from the macro, preserving behavior and doc examples.

Testing

• Ran cargo check -p uselesskey-rsa -p uselesskey-ecdsa -p uselesskey-ed25519 --all-features and it completed successfully.
• Ran formatting/lint checks with cargo xtask lint-fix --check --no-clippy and cargo clippy -p uselesskey-core -p uselesskey-rsa -p uselesskey-ecdsa -p uselesskey-ed25519 --all-features -- -D warnings with no warnings or failures.
• Ran the unit/test matrix cargo test -p uselesskey-core -p uselesskey-rsa -p uselesskey-ecdsa -p uselesskey-ed25519 --all-features and all tests passed.
• Note: an unrelated local change to fuzz/Cargo.lock pre-existed and was not included in this change set.

Codex Task