chore: prep Phase 3 tasks and workspace for alknet-secret development

- Add irpc (0.16) and irpc-derive (0.16) as workspace dependencies
- Add irpc, irpc-derive, and secp256k1 (optional) to alknet-secret Cargo.toml
- Clarify encryption-salt-kdf task: Option B (document salt as reserved) is the
  chosen path per spec update, removing Option A acceptance criteria
- Update irpc-secret-protocol-integration task with concrete irpc crate details:
  real crate on crates.io v0.16, #[rpc_requests] macro, workspace config,
  AuthProtocol pattern reference, DerivedKey serialization considerations
- Fix secp256k1-ethereum-derivation task: correct crate name is secp256k1
  (not libsecp256k1), add version pin 0.29

crates/alknet-secret/Cargo.toml

@@ -9,6 +9,10 @@ repository.workspace = true
 [lib]
 name = "alknet_secret"
 
+[features]
+default = []
+secp256k1 = ["dep:secp256k1"]
+
 [dependencies]
 bip39 = { version = "2", features = ["rand"] }
 ed25519-bip32 = "0.4"
@@ -21,6 +25,9 @@ zeroize = { version = "1", features = ["derive"] }
 hmac = "0.12"
 rand = "0.8"
 base64 = "0.22"
+irpc = { workspace = true }
+irpc-derive = { workspace = true }
+secp256k1 = { version = "0.29", optional = true }
 
 [dev-dependencies]
 hex = "0.4"