Add dev shell flake and justfile for local checks

Introduce a Nix dev shell and a justfile so work on this fork has a
reproducible toolchain and a one-command local gate, mirroring the setup
in the ldk-node repo.

The toolchain is pinned to stable 1.90.0 via fenix rather than tracking
latest. The version is bounded on both ends: clippy must be at least
1.87 so ci/check-lint.sh can resolve the lint names it allows (older
clippy hard-errors on the unknown clippy::manual_is_multiple_of), but
1.92+ adds clippy::assertions_on_constants which fires on existing code
in lightning/src/chain/channelmonitor.rs and would break -D warnings on
the inherited tree. 1.90.0 sits in that window and matches what CI's
stable resolves to today. fenix is used over rust-overlay because it
pins an exact channel by hash, so the shell can't drift.

just check only compiles and runs the workspace test suite. It
deliberately skips clippy and rustfmt: this branch is not clean under
either of CI's gates (pre-existing dead code in lsps2/lsps4 trips
check-lint.sh's -D warnings, and the tree predates the pinned rustfmt),
and reformatting or de-linting code we don't own is out of scope.

Author: amackillop

.envrc

@@ -0,0 +1 @@
+use flake

flake.lock

@@ -0,0 +1,85 @@
+{
+  description = "Rust Lightning Development Environment";
+
+  inputs = {
+    # Nixpkgs channel. New channels are released every 6 months.
+    # See: https://github.com/NixOS/nixpkgs/tags
+    nixpkgs.url = "github:nixos/nixpkgs/25.11";
+
+    # This makes it easy for the flake to be multi-platform.
+    # See: https://github.com/numtide/flake-utils
+    flake-utils.url = "github:numtide/flake-utils";
+
+    # Provides pinned Rust toolchains.
+    # See: https://github.com/nix-community/fenix
+    fenix = {
+      url = "github:nix-community/fenix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+      fenix,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+
+        # Pinned stable toolchain. The version is deliberately bounded:
+        #   * clippy must be >= 1.87 so `./ci/check-lint.sh` resolves the lints it
+        #     `-A`s (e.g. `clippy::manual_is_multiple_of`); older clippy E0602s on the
+        #     unknown lint name.
+        #   * but NOT 1.92+, whose clippy grows `assertions_on_constants` (fires on
+        #     `assert!(cfg!(fuzzing), ..)` in lightning/src/chain/channelmonitor.rs)
+        #     and would break `-D warnings` on the pre-existing tree.
+        # 1.90.0 sits in that window and matches what CI's `stable` resolves to.
+        rust-toolchain =
+          (fenix.packages.${system}.toolchainOf {
+            channel = "1.90.0";
+            sha256 = "sha256-SJwZ8g0zF2WrKDVmHrVG3pD2RGoQeo24MEXnNx5FyuI=";
+          }).withComponents
+            [
+              "rustc"
+              "cargo"
+              "clippy"
+              "rustfmt"
+              "rust-src" # Needed for the rust-analyzer extension to work.
+            ];
+      in
+      {
+        # The default development shell. Use `nix develop` or direnv to enter it.
+        devShells.default = pkgs.mkShell {
+          name = "rust-lightning-dev-shell";
+
+          packages = [
+            rust-toolchain
+          ]
+          ++ (with pkgs; [
+            just # Command runner
+            nodejs # JavaScript runtime, required for MCP tools
+            mold # Fast linker for Rust/C/C++
+            pnpm # Package manager for JavaScript, required for MCP tools
+            stdenv.cc.cc.lib # C++ standard library for runtime
+          ]);
+
+          env = {
+            # OpenSSL configuration for Nix
+            PKG_CONFIG_PATH = "${pkgs.openssl.dev}/lib/pkgconfig";
+            # C++ standard library path for runtime
+            LD_LIBRARY_PATH = "${pkgs.stdenv.cc.cc.lib}/lib";
+          };
+
+          shellHook = ''
+            echo "Rust Lightning dev shell"
+            rustc --version
+            cargo --version
+          '';
+        };
+      }
+    );
+}

flake.nix

@@ -0,0 +1,14 @@
+default:
+  @just --list --unsorted
+
+# Run all code quality checks. TODO: Add format and linting when branch is clean
+check:
+  RUSTFLAGS="--cfg=lsps1_service" cargo test --workspace
+
+# Run just the tests
+test:
+  RUSTFLAGS="--cfg=lsps1_service" cargo test --workspace
+
+# Format all sources in place.
+fmt:
+  cargo fmt --all