refactor(halo2): Decouple circuit from Midnight types

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).
 
+## 0.9.27 (03-11-2026)
+
+### Changed
+
+- Introduced circuit-local Halo2 types in `circuits/halo2/types.rs` with shared `CircuitBaseField`, `CircuitBase`, and `CircuitCurve`.
+- Replaced duplicated local `F`/`C` aliases with shared circuit types across Halo2 circuit, gadgets, and golden helper code.
+- Standardized Halo2 conversion paths using `From`/`Into` implementations for circuit/domain field wrappers.
+- Added `circuits/halo2/adapters.rs` to convert STM Merkle paths into Halo2 witness paths for circuit consumption.
+- Removed `circuits/halo2/utils/mod.rs` and inlined field-limb split logic into `circuits/halo2/gadgets.rs`.
+- Unified synthesis error mapping through `to_synthesis_error` in `circuits/halo2/errors.rs`.
+
 ## 0.9.26 (03-09-2026)
 
 ### Changed

mithril-stm/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mithril-stm"
-version = "0.9.26"
+version = "0.9.27"
 edition = { workspace = true }
 authors = { workspace = true }
 homepage = { workspace = true }

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

@@ -0,0 +1,48 @@
+//! Adapters for converting STM-side structures to Halo2 circuit witness structures.
+
+use digest::Digest;
+use thiserror::Error;
+
+use crate::circuits::halo2::types::{MerklePath as Halo2MerklePath, Position};
+use crate::membership_commitment::MerklePath as StmMerklePath;
+use crate::signature_scheme::BaseFieldElement;
+
+/// Errors returned when adapting STM Merkle paths to Halo2 witness paths.
+#[derive(Debug, Error)]
+pub enum MerklePathAdapterError {
+    #[error("invalid merkle digest length")]
+    InvalidDigestLength,
+    #[error("non-canonical merkle digest")]
+    NonCanonicalDigest,
+}
+
+impl<D: Digest> TryFrom<&StmMerklePath<D>> for Halo2MerklePath {
+    type Error = MerklePathAdapterError;
+
+    fn try_from(stm_path: &StmMerklePath<D>) -> Result<Self, Self::Error> {
+        let mut siblings = Vec::with_capacity(stm_path.values.len());
+
+        for (i, value) in stm_path.values.iter().enumerate() {
+            let bytes: [u8; 32] = value
+                .as_slice()
+                .try_into()
+                .map_err(|_| MerklePathAdapterError::InvalidDigestLength)?;
+            let node = BaseFieldElement::from_bytes(&bytes)
+                .ok()
+                .map(|base| base.into())
+                .ok_or(MerklePathAdapterError::NonCanonicalDigest)?;
+            let bit = (stm_path.index >> i) & 1;
+            // At level `i`, `bit = (index >> i) & 1`: `0` means current is left, `1` means right.
+            // STM uses `H(current || sibling)` for `bit == 0`, else `H(sibling || current)`.
+            // Map `0 -> Position::Right` and `1 -> Position::Left` so Halo2 folds identically.
+            let position = if bit == 0 {
+                Position::Right
+            } else {
+                Position::Left
+            };
+            siblings.push((position, node));
+        }
+
+        Ok(Halo2MerklePath::new(siblings))
+    }
+}