aes-gcm 0.11: key and nonce generation move to the Generate trait

[stoatworks-labs]

Supersedes #15, which cannot compile: aes-gcm 0.11 removes aead::OsRng and the generate_key / generate_nonce associated functions.

error[E0432]: unresolved import `aes_gcm::aead::OsRng`
error[E0599]: no associated function named `generate_nonce` found for struct `AesGcm<...>`

RustCrypto moved key and nonce generation onto a Generate trait (crypto-common 0.2), so the calls become:

before	after
Aes256Gcm::generate_key(OsRng)	Key::<Aes256Gcm>::generate()
Aes256Gcm::generate_nonce(OsRng)	Nonce::<<Aes256Gcm as AeadCore>::NonceSize>::generate()

Why this should be equivalent, and where to check me. Generate::generate() draws from the system's ambient CSPRNG (SysRng → getrandom), which is what OsRng was. The nonce type is derived from AeadCore::NonceSize rather than written as a literal, so it stays 12 bytes — the existing NONCE_LEN assertion on the decrypt path still passes. Key size is unchanged at 256 bits.

One behavioural note: generate() panics if the OS RNG fails, where the old OsRng path did the same. try_generate() exists if a Result is preferred.

Verified: cargo build --all-targets clean, and the crypto tests pass — round-trip, key-file persistence across reload, and the legacy-plaintext passthrough.

⚠️ Not self-merging. This is key and nonce generation, and "it compiles and the tests are green" is weak evidence for crypto — a test suite will not notice a weakened RNG or a nonce-size change. Wants a human read before it lands.