crypto: bound age subprocess lifetime

[fosskar]

Summary

• resolve age and age-keygen executable paths once when constructing AgeIdentity
• drive child stdin/stdout asynchronously with an inactivity timeout and guaranteed child cleanup
• preserve per-operation 0600 identity files while removing avoidable plaintext/ciphertext copies

Verification

• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• focused injected-process tests cover empty environments, inactivity timeout, child reaping, executable resolution, and identity-file lifetime
• release benchmark stayed within the 5% throughput gate with flat peak RSS

Stacked on deployment-storage-hardening.