refactor(stm): cache the circuit test fixtures on disk

mithril-common/Cargo.toml

@@ -51,7 +51,7 @@ fixed = "1.31.0"
 hex = { workspace = true }
 kes-summed-ed25519 = { version = "0.2.1", features = ["serde_enabled", "sk_clone_enabled"] }
 mithril-merkle-tree = { path = "../internal/mithril-merkle-tree", version = "0.1.4" }
-mithril-stm = { path = "../mithril-stm", version = "0.12.7", default-features = false }
+mithril-stm = { path = "../mithril-stm", version = "0.12.8", default-features = false }
 nom = "8.0.0"
 rand_chacha = { workspace = true }
 rand_core = { workspace = true }

mithril-stm/CHANGELOG.md

@@ -5,6 +5,17 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## 0.12.8 (08-20-2026)
+
+### Changed
+
+- Extended the on-disk test cache to the recursive circuit fixtures, so the recursive verifying key, the certificate golden circuit keys and the deterministic signer fixture are derived once and shared across test processes instead of being rebuilt by each one.
+- Shared the unsafe SRS cache between the circuit test generators and the prover setup, so the SRS is generated once per degree across the whole test suite.
+
+### Removed
+
+- Removed the `StmCircuitError::CircuitKeysCacheLockPoisoned` variant, which became unreachable once the in-process circuit key cache was replaced by the on-disk cache.
+
 ## 0.12.7 (08-19-2026)
 
 ### Changed

mithril-stm/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mithril-stm"
-version = "0.12.7"
+version = "0.12.8"
 edition = { workspace = true }
 authors = { workspace = true }
 homepage = { workspace = true }

mithril-stm/src/circuits/halo2/errors.rs

@@ -101,10 +101,6 @@ pub enum StmCircuitError {
     #[error("Failed to create params assets directory")]
     ParamsAssetsDirCreate,
 
-    /// In-memory circuit key cache lock is poisoned.
-    #[error("Circuit keys cache lock poisoned ({operation})")]
-    CircuitKeysCacheLockPoisoned { operation: &'static str },
-
     /// Signature generation failed while preparing witness.
     #[error("Signature generation failed")]
     SignatureGenerationFailed,

mithril-stm/src/circuits/halo2/keys.rs

@@ -76,11 +76,6 @@ mod midnight_verifying_key_serde {
 }
 
 impl NonRecursiveCircuitProvingKey {
-    /// Wraps a Midnight proving key.
-    pub(crate) fn new(midnight_pk: MidnightPK<StmCertificateCircuit>) -> Self {
-        Self(midnight_pk)
-    }
-
     /// Borrows the wrapped Midnight proving key, for proof generation.
     pub(crate) fn midnight_pk(&self) -> &MidnightPK<StmCertificateCircuit> {
         &self.0