feat(tbor): implement AesGenerateKey + AesEncryptDecrypt (masked-key AES)

[vsonims]

Summary

Adds the TBOR AES crypto command pair, stacked on the RSA-AES
key-unwrap commands (#583), following the MBOR AES commands but with the
TBOR masked-key (stateless) model — no vault key_id / key_tag.

AesGenerateKey (opcode 0x15)

Generate a fresh random AES key (128 / 192 / 256 bits, non-bulk only,
mirroring MBOR) and return it masked (AEAD-GCM-256) under the
requested scope's masking key. The masked-blob length is fixed by the key
size, so the handler reserves the response slot up front and masks the
generated key straight into it (encoder *_reserve + decode_mut) — no
scratch buffer, no copy.

AesEncryptDecrypt (opcode 0x16)

AES-CBC encrypt or decrypt a host-supplied message with a caller-held
masked AES key. Fully zero-copy:

• request masked_key is #[tbor(mutable)] → the handler decode_muts
  the request and unmasks the blob in place, using the recovered
  key directly (no blob / key scratch copy);
• the response msg / iv slots are reserved, and the non-in-place
  aes_cbc_enc_dec reads the request message and writes the ciphertext /
  chaining IV straight into the response (no message copy).

Rejects non-AES key kinds (InvalidKeyType) and direction-permission
mismatches (InvalidPermissions); the recovered key is wiped on every
path (including the unmask-failure path).

AES key import

Already provided by UnwrapKey (0x14, key class Aes) in #583 — an
emu test exercises the UnwrapKey → AesEncryptDecrypt round-trip.

Wiring

Both opcodes are routed through the fw dispatcher, the is_known_opcode
/ is_in_session / needs_session_id_cross_check classifiers, and
op::SessionCtrl::from_tbor_opcode (InSession, CO/CU). Reuses the shared
validate_active_session / resolve_masking_key helpers and the encoder
reserve/fill support from #583.

Tests / validation

• Emu: keygen round-trip across all sizes/scopes, before-finalize +
  SD-scope + unknown-size rejects; encrypt→decrypt round-trip all sizes,
  CBC chaining IV, tamper + bad-length rejects, and the UnwrapKey→AES
  cross-command round-trip.
• Full TBOR emu suite green (127 tests), clippy clean, nightly
  fmt + copyright clean, Uno (thumbv7em) build passes.
• Docs: docs/tbor-ddi/commands/{aes_generate_key,aes_encrypt_decrypt}.md
  • README rows (0x15, 0x16).