test(storage): SecureStorage static crypto helpers (+14 tests)

[TaprootFreak]

Summary

Stage 29 of the coverage push. Pure-Dart static methods on `SecureStorage` — pinning the on-disk crypto contract for PIN-hashing and seed-encryption.

Helper	Cases
`generatePinSalt`	3
`hashPin`	5
`encryptSeed / decryptSeed` round-trip	6

What's pinned

• generatePinSalt: returns a 16-byte `Uint8List`; consecutive calls return distinct salts (CSPRNG); every byte falls in 0..255.
• hashPin: returns a 64-char lowercase-hex string (32 bytes); deterministic for the same `(pin, salt, iterations)` triple; sensitive to each axis individually — including the iteration-count variation that exists for the legacy-migration path (10k → 600k).
• encryptSeed / decryptSeed: round-trips a typical 12-word mnemonic, an empty string, and unicode content; GCM nonce randomisation gives distinct ciphertexts for the same plaintext (privacy property); decrypt with the wrong key throws; output format is pinned as `:` (load-bearing for storage forward-compat).

Notes

• Tests use `iterations: 1` on the synchronous `hashPin` to avoid the 600k-iter PBKDF2 cost. The deterministic / sensitivity properties hold for any iteration count.
• These helpers are exercised indirectly in the existing PIN cubit tests, but a direct contract for them lets a crypto-edge-case regression (e.g. someone "optimising" the IV size) surface immediately.

Test plan

• `flutter analyze` clean
• `flutter test` — 14 / 14 passing locally
• CI green