rename crates/ to engrams/, bindings/ to receptors/

- crates/ → engrams/ (Rust engrams live here)
- bindings/ → receptors/ (cross-language access points into the graph)
- Cargo.toml workspace paths updated

engrams/engram-crypto/src/error.rs

@@ -0,0 +1,30 @@
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum CryptoError {
+    #[error("Encryption failed: {0}")]
+    EncryptionFailed(String),
+
+    #[error("Decryption failed: {0}")]
+    DecryptionFailed(String),
+
+    #[error("Signature verification failed")]
+    SignatureInvalid,
+
+    #[error("Unknown algorithm ID: {0}")]
+    UnknownAlgorithm(String),
+
+    #[error("Key derivation failed: {0}")]
+    KeyDerivation(String),
+
+    #[error("Invalid key length: expected {expected}, got {got}")]
+    InvalidKeyLength { expected: usize, got: usize },
+
+    #[error("Algorithm not available: {0}")]
+    AlgorithmUnavailable(String),
+
+    #[error("Serialization error: {0}")]
+    Serialization(String),
+}
+
+pub type CryptoResult<T> = Result<T, CryptoError>;