build(solidity): raise hardhat to 2.19.5 and pin smock, in both packages

.gitignore

@@ -79,3 +79,6 @@ venv/
 target/
 dist/
 .DS_Store
+
+# Yarn 4 local state (not for zero-installs)
+.yarn/install-state.gz

solidity/ecdsa/contracts/test/IMockTarget.sol

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-3.0-only
+
+pragma solidity ^0.8.0;
+
+/// @notice Test-only interface exercising the shapes `MockContract` has to
+///         answer: a view function reached by STATICCALL, a state-changing
+///         function whose calls are asserted on, a function returning nothing,
+///         a struct return and a multi-value return.
+interface IMockTarget {
+    struct Info {
+        address owner;
+        uint64 createdAt;
+        bool active;
+    }
+
+    function doThing(address who, uint256 amount) external returns (bool);
+
+    function noReturn(uint256 value) external;
+
+    function readValue(uint256 key) external view returns (uint256);
+
+    function readInfo(uint256 key) external view returns (Info memory);
+
+    function readPair(uint256 key) external view returns (uint32, uint32);
+}