Add Nix dev shell with pinned test binaries

.envrc

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+use flake

flake.nix

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+{
+  description = "LDK Node 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 Rust toolchains.
+    # See: https://github.com/oxalica/rust-overlay
+    rust-overlay.url = "github:oxalica/rust-overlay";
+
+    # Pinned nixpkgs that ships bitcoind 27.1. The integration tests' bundled
+    # `corepc-node` deserializes the 27.x `getblockchaininfo` schema, so newer
+    # bitcoind (25.11 ships 30.0) fails RPC decoding. Used only for BITCOIND_EXE.
+    nixpkgs-bitcoind.url = "github:nixos/nixpkgs/ab7b6889ae9d484eed2876868209e33eb262511d";
+  };
+
+  outputs =
+    {
+      self,
+      nixpkgs,
+      flake-utils,
+      rust-overlay,
+      nixpkgs-bitcoind,
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        # Overlays provide additional packages not available in the channels.
+        overlays = [
+          # Provides the rust-bin package; a set of pre-built Rust toolchains.
+          (import rust-overlay)
+        ];
+
+        # The final set of packages.
+        pkgs = import nixpkgs {
+          # Inheriting from system is what makes this multi-platform.
+          # We also inherit the overlays that we want to use.
+          inherit system overlays;
+        };
+
+        # bitcoind 27.1 from the pinned input (see inputs above).
+        bitcoind = nixpkgs-bitcoind.legacyPackages.${system}.bitcoind;
+
+        # The specific Rust toolchain that we use in development shells.
+        # Matches the `rust-version` declared in Cargo.toml. We use the `minimal`
+        # profile (rustc, cargo, rust-std) plus clippy and rust-src, but
+        # deliberately exclude `rustfmt`: this repo's `rustfmt.toml` relies on
+        # nightly-only options, so formatting is supplied by `rustfmt-nightly`
+        # below to keep `just fmt`/`just check` consistent with CI's nightly job.
+        rust-toolchain = pkgs.rust-bin.stable."1.85.1".minimal.override {
+          extensions = [
+            "rust-src" # Needed for the rust-analyzer extension to work.
+            "clippy" # Linter used by `just check`.
+          ];
+        };
+
+        # Nightly rustfmt only. `cargo fmt` shells out to whichever `rustfmt` is
+        # on PATH, so a nightly rustfmt lets the nightly-only options in
+        # `rustfmt.toml` apply even though cargo/rustc are pinned to stable.
+        rustfmt-nightly = pkgs.rust-bin.nightly.latest.rustfmt;
+
+        # Esplora/HTTP electrs for the integration tests' Esplora chain source.
+        #
+        # We deliberately do NOT use nixpkgs' `blockstream-electrs`: it is a far
+        # newer build whose initial regtest indexing takes ~80s, blowing past the
+        # tests' sync timeout. Instead we pin the *exact* prebuilt binary CI uses
+        # (see scripts/download_bitcoind_electrs.sh) and patch it to run on Nix.
+        # This keeps local test behaviour identical to CI.
+        electrs-esplora =
+          let
+            rev = "a33e97e1a1fc63fa9c20a116bb92579bbf43b254";
+            sources = {
+              x86_64-linux = {
+                file = "electrs_linux_esplora_${rev}.zip";
+                sha256 = "865e26a96e8df77df01d96f2f569dcf9622fc87a8d99a9b8fe30861a4db9ddf1";
+              };
+              x86_64-darwin = {
+                file = "electrs_macos_esplora_${rev}.zip";
+                sha256 = "2d5ff149e8a2482d3658e9b386830dfc40c8fbd7c175ca7cbac58240a9505bcd";
+              };
+            };
+            src =
+              sources.${system}
+                or (throw "no prebuilt esplora electrs for ${system}");
+          in
+          pkgs.stdenv.mkDerivation {
+            pname = "electrs-esplora";
+            version = "esplora-${builtins.substring 0 9 rev}";
+            src = pkgs.fetchurl {
+              url = "https://github.com/RCasatta/electrsd/releases/download/electrs_releases/${src.file}";
+              inherit (src) sha256;
+            };
+            nativeBuildInputs =
+              [ pkgs.unzip ] ++ pkgs.lib.optional pkgs.stdenv.isLinux pkgs.autoPatchelfHook;
+            buildInputs = pkgs.lib.optionals pkgs.stdenv.isLinux [ pkgs.stdenv.cc.cc.lib ];
+            unpackPhase = ''
+              runHook preUnpack
+              unzip "$src"
+              runHook postUnpack
+            '';
+            installPhase = ''
+              runHook preInstall
+              install -Dm755 electrs "$out/bin/electrs"
+              runHook postInstall
+            '';
+          };
+
+      in
+      {
+        # The development shell. Use `nix develop` or direnv to enter it.
+        devShells.default = pkgs.mkShell {
+          name = "ldk-node-dev-shell";
+
+          packages = with pkgs; [
+            rustfmt-nightly # Nightly rustfmt; must precede the stable toolchain on PATH.
+            rust-toolchain # Rust toolchain (no rustfmt; see above).
+            nodejs # JavaScript runtime, required for MCP tools
+            mold # Fast linker for Rust/C/C++
+            pnpm # Package manager for JavaScript, required for MCP tools
+            pkg-config # Required by build scripts that link against system libraries
+            just # Command runner used for `just check`, `just fmt`, etc.
+            stdenv.cc.cc.lib # C++ standard library for runtime
+          ];
+
+          env = {
+            # OpenSSL configuration for Nix
+            PKG_CONFIG_PATH = "${pkgs.openssl.dev}/lib/pkgconfig";
+            # C++ standard library path for runtime
+            LD_LIBRARY_PATH = "${pkgs.stdenv.cc.cc.lib}/lib";
+            # Integration tests (run with `--cfg no_download`) locate these via env
+            # rather than downloading generic-linux binaries that can't run on NixOS.
+            BITCOIND_EXE = "${bitcoind}/bin/bitcoind";
+            ELECTRS_EXE = "${electrs-esplora}/bin/electrs";
+          };
+
+          shellHook = ''
+            echo "LDK Node dev shell"
+            rustc --version
+            cargo --version
+          '';
+        };
+      }
+    );
+}

justfile

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+default:
+  @just --list --unsorted
+
+# Validate: rustfmt check + clippy with warnings denied (matches CI's deny-warnings gate).
+check:
+  cargo fmt --all -- --check
+  cargo clippy --all-targets -- -D warnings
+
+# Format all sources in place.
+fmt:
+  cargo fmt --all
+
+# Apply clippy autofixes across all targets.
+fix:
+  cargo clippy --all-targets --fix --allow-dirty --allow-staged
+
+# Run library unit tests only. No bitcoind/electrs required.
+test:
+  RUSTFLAGS="-D warnings" cargo test --lib
+
+# Run full suite incl. integration tests. Needs bitcoind+electrs on PATH (CI's --cfg no_download).
+test-all:
+  RUSTFLAGS="--cfg no_download -D warnings" cargo test