refactor(cli): single canonical ParamsFile in session.rs (Plan B Phase 5)

Promotes ParamsFile to a module-level pub(crate) struct with both Serialize
and Deserialize derives. for_new_vault() constructor + into_kdf_params()
inversion replace the two-definition split between commands/init.rs (write)
and session.rs read_params (read). On-disk JSON format unchanged — fixture
test asserts round-trip with the current params.json layout.

crates/relicario-cli/src/session.rs

@@ -107,17 +107,52 @@ fn read_salt(root: &Path) -> Result<[u8; 32]> {
     Ok(salt)
 }
 
-fn read_params(root: &Path) -> Result<KdfParams> {
-    // params.json layout: { "format_version": 2, "kdf": { "argon2_m": ..., ... }, ... }
-    // We extract only the "kdf" sub-object and deserialize it as KdfParams.
-    #[derive(serde::Deserialize)]
-    struct ParamsFile {
-        kdf: KdfParams,
+#[derive(serde::Serialize, serde::Deserialize)]
+pub(crate) struct ParamsFile {
+    pub format_version: u32,
+    pub kdf: ParamsKdf,
+    pub aead: String,
+    pub salt_path: String,
+}
+
+#[derive(serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub(crate) struct ParamsKdf {
+    pub algorithm: String,
+    pub argon2_m: u32,
+    pub argon2_t: u32,
+    pub argon2_p: u32,
+}
+
+impl ParamsFile {
+    pub fn for_new_vault(params: &KdfParams) -> Self {
+        Self {
+            format_version: 2,
+            kdf: ParamsKdf {
+                algorithm: "argon2id-v0x13".into(),
+                argon2_m: params.argon2_m,
+                argon2_t: params.argon2_t,
+                argon2_p: params.argon2_p,
+            },
+            aead: "xchacha20poly1305".into(),
+            salt_path: ".relicario/salt".into(),
+        }
     }
+
+    pub fn into_kdf_params(self) -> KdfParams {
+        KdfParams {
+            argon2_m: self.kdf.argon2_m,
+            argon2_t: self.kdf.argon2_t,
+            argon2_p: self.kdf.argon2_p,
+        }
+    }
+}
+
+fn read_params(root: &Path) -> Result<KdfParams> {
     let s = fs::read_to_string(root.join(".relicario").join("params.json"))
         .context("failed to read .relicario/params.json")?;
     let pf: ParamsFile = serde_json::from_str(&s).context("failed to parse params.json")?;
-    Ok(pf.kdf)
+    Ok(pf.into_kdf_params())
 }
 
 /// Locate the reference image path via `RELICARIO_IMAGE` env var or interactive prompt.
@@ -149,3 +184,72 @@ fn atomic_write(path: &Path, data: &[u8]) -> Result<()> {
     fs::rename(&tmp, path).with_context(|| format!("failed to rename {}", path.display()))?;
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const FIXTURE: &str = r#"{
+  "format_version": 2,
+  "kdf": {
+    "algorithm": "argon2id-v0x13",
+    "argon2_m": 65536,
+    "argon2_t": 3,
+    "argon2_p": 4
+  },
+  "aead": "xchacha20poly1305",
+  "salt_path": ".relicario/salt"
+}"#;
+
+    #[test]
+    fn params_file_round_trips_current_layout() {
+        let pf: ParamsFile = serde_json::from_str(FIXTURE).expect("parse fixture");
+        assert_eq!(pf.format_version, 2);
+        assert_eq!(pf.kdf.algorithm, "argon2id-v0x13");
+        assert_eq!(pf.kdf.argon2_m, 65536);
+        assert_eq!(pf.kdf.argon2_t, 3);
+        assert_eq!(pf.kdf.argon2_p, 4);
+        assert_eq!(pf.aead, "xchacha20poly1305");
+        assert_eq!(pf.salt_path, ".relicario/salt");
+
+        let kdf = ParamsFile {
+            format_version: pf.format_version,
+            kdf: ParamsKdf {
+                algorithm: pf.kdf.algorithm.clone(),
+                argon2_m: pf.kdf.argon2_m,
+                argon2_t: pf.kdf.argon2_t,
+                argon2_p: pf.kdf.argon2_p,
+            },
+            aead: pf.aead.clone(),
+            salt_path: pf.salt_path.clone(),
+        }
+        .into_kdf_params();
+        assert_eq!(kdf.argon2_m, 65536);
+        assert_eq!(kdf.argon2_t, 3);
+        assert_eq!(kdf.argon2_p, 4);
+
+        let serialized = serde_json::to_string(&pf).expect("re-serialize");
+        let pf2: ParamsFile = serde_json::from_str(&serialized).expect("parse re-serialized");
+        assert_eq!(pf2.format_version, 2);
+        assert_eq!(pf2.kdf.algorithm, "argon2id-v0x13");
+        assert_eq!(pf2.kdf.argon2_m, 65536);
+        assert_eq!(pf2.kdf.argon2_t, 3);
+        assert_eq!(pf2.kdf.argon2_p, 4);
+        assert_eq!(pf2.aead, "xchacha20poly1305");
+        assert_eq!(pf2.salt_path, ".relicario/salt");
+    }
+
+    #[test]
+    fn for_new_vault_produces_expected_shape() {
+        let params = KdfParams { argon2_m: 65536, argon2_t: 3, argon2_p: 4 };
+        let pf = ParamsFile::for_new_vault(&params);
+        let v = serde_json::to_value(&pf).expect("to_value");
+        assert_eq!(v["format_version"], 2);
+        assert_eq!(v["kdf"]["algorithm"], "argon2id-v0x13");
+        assert_eq!(v["kdf"]["argon2_m"], 65536);
+        assert_eq!(v["kdf"]["argon2_t"], 3);
+        assert_eq!(v["kdf"]["argon2_p"], 4);
+        assert_eq!(v["aead"], "xchacha20poly1305");
+        assert_eq!(v["salt_path"], ".relicario/salt");
+    }
+}