add interface for immutable state

src/base/ImmutableState.sol

@@ -2,11 +2,12 @@
 pragma solidity ^0.8.0;
 
 import {IPoolManager} from "@uniswap/v4-core/src/interfaces/IPoolManager.sol";
+import {IImmutableState} from "../interfaces/IImmutableState.sol";
 
 /// @title Immutable State
 /// @notice A collection of immutable state variables, commonly used across multiple contracts
-contract ImmutableState {
-    /// @notice The Uniswap v4 PoolManager contract
+contract ImmutableState is IImmutableState {
+    /// @inheritdoc IImmutableState
     IPoolManager public immutable poolManager;
 
     constructor(IPoolManager _poolManager) {

src/interfaces/IImmutableState.sol

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity ^0.8.0;
+
+import {IPoolManager} from "@uniswap/v4-core/src/interfaces/IPoolManager.sol";
+
+/// @title Interface for ImmutableState
+interface IImmutableState {
+    /// @notice The Uniswap v4 PoolManager contract
+    function poolManager() external view returns (IPoolManager);
+}

src/interfaces/IPositionManager.sol

@@ -5,10 +5,11 @@ import {PoolKey} from "@uniswap/v4-core/src/types/PoolKey.sol";
 import {PositionInfo} from "../libraries/PositionInfoLibrary.sol";
 
 import {INotifier} from "./INotifier.sol";
+import {IImmutableState} from "./IImmutableState.sol";
 
 /// @title IPositionManager
 /// @notice Interface for the PositionManager contract
-interface IPositionManager is INotifier {
+interface IPositionManager is INotifier, IImmutableState {
     /// @notice Thrown when the caller is not approved to modify a position
     error NotApproved(address caller);
     /// @notice Thrown when the block.timestamp exceeds the user-provided deadline

src/interfaces/IV4Router.sol

@@ -4,10 +4,11 @@ pragma solidity ^0.8.0;
 import {PoolKey} from "@uniswap/v4-core/src/types/PoolKey.sol";
 import {Currency} from "@uniswap/v4-core/src/types/Currency.sol";
 import {PathKey} from "../libraries/PathKey.sol";
+import {IImmutableState} from "./IImmutableState.sol";
 
 /// @title IV4Router
 /// @notice Interface containing all the structs and errors for different v4 swap types
-interface IV4Router {
+interface IV4Router is IImmutableState {
     /// @notice Emitted when an exactInput swap does not receive its minAmountOut
     error V4TooLittleReceived(uint256 minAmountOutReceived, uint256 amountReceived);
     /// @notice Emitted when an exactOutput is asked for more than its maxAmountIn