fix: recover v3 recipients from provenance seed

[mm9942]

Summary

• add a public 32-byte provenance-seed derivation API for v3 recipient recovery
• rederive recipient key pairs on open instead of treating KEM-specific private material as provenance state
• release 3.0.2 across manifests and README; remove the unreachable draft reference

Root cause

The v3 recipient representation can use KEM-specific private material larger than Harw's fixed 32-byte provenance seed. Persisting that representation crossed the provenance boundary incorrectly.

Validation

• cargo fmt --check
• git diff --check
• CARGO_HOME=/tmp/crypt-guard-cargo-home cargo --config 'build.rustc-wrapper=""' test
• CARGO_HOME=/tmp/crypt-guard-cargo-home cargo --config 'build.rustc-wrapper=""' package --manifest-path crypt_guard_proc/Cargo.toml --allow-dirty --offline