feat: add persistent PQ key storage

[knoal]

Closes #36.

Summary

• add switchboard/pq_keys.py with PQKeyPair.generate/load/save/sign and module-level verify
• derive stable key_id values from sha256(pk)[:16].hex()
• persist private keys in a PEM-like envelope using scrypt + chacha20-poly1305
• write companion .pub files with algorithm and key-id metadata
• keep the module import-safe without liboqs; only generation/signing/verification depend on PQ runtime support
• add focused tests for round-trip persistence, wrong-passphrase failure, deterministic key IDs, and delegation behavior

Validation

• python3 -m compileall switchboard/pq_keys.py tests/test_pq_keys.py
• python3 -m pytest tests/test_pq_keys.py tests/test_pq.py -v

Notes

• tests/test_pq_keys.py passes fully in this environment
• the behavioral tests/test_pq.py cases that require liboqs-python remain skipped here because oqs is not installed; the always-on structural/no-liboqs coverage still passed