Add Nix flake for development environment (#23406)

Provides a Nix flake as an alternative to `scripts/bootstrap.sh` for
setting up the Bun development environment.

## What's included:

- **flake.nix**: Full development environment with all dependencies from
bootstrap.sh
  - LLVM 19, CMake 3.30+, Node.js 24, Rust, Go
  - Build tools: ninja, ccache, pkg-config, make
  - Chromium dependencies for Puppeteer testing
  - gdb for core dump debugging

- **shell.nix**: Simple wrapper for `nix-shell` usage

- **cmake/CompilerFlags.cmake**: Nix compatibility fixes
  - Disable zstd debug compression (Nix's LLVM not built with zstd)
  - Set _FORTIFY_SOURCE=0 for -O0 debug builds
  - Downgrade _FORTIFY_SOURCE warning to not error

## Usage:

```bash
nix-shell
export CMAKE_SYSTEM_PROCESSOR=$(uname -m)
bun bd
```

## Verified working:
✅ Successfully compiles Bun debug build
✅ Binary tested: `./build/debug/bun-debug --version` → 1.2.24-debug
✅ All dependencies from bootstrap.sh included

## Advantages:
- Fully isolated (no sudo required)
- 100% reproducible dependency versions  
- Fast setup with binary caching

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>

shell.nix

@@ -0,0 +1,103 @@
+# Simple shell.nix for users without flakes enabled
+# For reproducible builds with locked dependencies, use: nix develop
+# This uses unpinned <nixpkgs> for simplicity; flake.nix provides version pinning via flake.lock
+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.mkShell rec {
+  packages = with pkgs; [
+    # Core build tools (matching bootstrap.sh)
+    cmake
+    ninja
+    clang_19
+    llvm_19
+    lld_19
+    nodejs_24
+    bun
+    rustc
+    cargo
+    go
+    python3
+    ccache
+    pkg-config
+    gnumake
+    libtool
+    ruby
+    perl
+
+    # Libraries
+    openssl
+    zlib
+    libxml2
+
+    # Development tools
+    git
+    curl
+    wget
+    unzip
+    xz
+
+    # Linux-specific: gdb and Chromium deps for testing
+  ] ++ pkgs.lib.optionals pkgs.stdenv.isLinux [
+    gdb
+    # Chromium dependencies for Puppeteer tests
+    xorg.libX11
+    xorg.libxcb
+    xorg.libXcomposite
+    xorg.libXcursor
+    xorg.libXdamage
+    xorg.libXext
+    xorg.libXfixes
+    xorg.libXi
+    xorg.libXrandr
+    xorg.libXrender
+    xorg.libXScrnSaver
+    xorg.libXtst
+    libxkbcommon
+    mesa
+    nspr
+    nss
+    cups
+    dbus
+    expat
+    fontconfig
+    freetype
+    glib
+    gtk3
+    pango
+    cairo
+    alsa-lib
+    at-spi2-atk
+    at-spi2-core
+    libgbm
+    liberation_ttf
+    atk
+    libdrm
+    xorg.libxshmfence
+    gdk-pixbuf
+  ];
+
+  shellHook = ''
+    export CC="${pkgs.lib.getExe pkgs.clang_19}"
+    export CXX="${pkgs.lib.getExe' pkgs.clang_19 "clang++"}"
+    export AR="${pkgs.llvm_19}/bin/llvm-ar"
+    export RANLIB="${pkgs.llvm_19}/bin/llvm-ranlib"
+    export CMAKE_C_COMPILER="$CC"
+    export CMAKE_CXX_COMPILER="$CXX"
+    export CMAKE_AR="$AR"
+    export CMAKE_RANLIB="$RANLIB"
+    export CMAKE_SYSTEM_PROCESSOR=$(uname -m)
+    export TMPDIR=''${TMPDIR:-/tmp}
+  '' + pkgs.lib.optionalString pkgs.stdenv.isLinux ''
+    export LD="${pkgs.lib.getExe' pkgs.lld_19 "ld.lld"}"
+    export NIX_CFLAGS_LINK="''${NIX_CFLAGS_LINK:+$NIX_CFLAGS_LINK }-fuse-ld=lld"
+    export LD_LIBRARY_PATH="${pkgs.lib.makeLibraryPath packages}''${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}"
+  '' + ''
+
+    echo "====================================="
+    echo "Bun Development Environment (Nix)"
+    echo "====================================="
+    echo "To build: bun bd"
+    echo "To test:  bun bd test <test-file>"
+    echo "====================================="
+  '';
+}