feat: Add ARM64 Linux support (aarch64-unknown-linux-gnu)

[akaoio]

Summary

This PR adds comprehensive ARM64 (aarch64) support for Linux systems, enabling @replit/ruspty to work on ARM64 devices like Raspberry Pi, AWS Graviton, and Apple Silicon Linux VMs.

Changes

• ✅ Added aarch64-unknown-linux-gnu to supported platforms in package.json
• ✅ Created npm package configuration for ARM64 Linux binary
• ✅ Made sandbox module x86_64-specific (uses architecture-specific CPU registers)
• ✅ Added conditional compilation for x86_64-only features
• ✅ Added graceful fallback with warning for unsupported architectures
• ✅ Added comprehensive test suite for ARM64 verification
• ✅ Added detailed documentation and build instructions

Technical Details

The sandbox module uses x86_64-specific CPU registers (rsi, rdx) for seccomp filtering, so it's been made conditional for x86_64 only with:

#[cfg(all(target_os = "linux", target_arch = "x86_64"))]
mod sandbox;

This is a non-critical feature and doesn't affect core PTY functionality on ARM64.

Testing

Successfully tested on ARM64 Linux (Orange Pi 5 Plus) with all core PTY features working:

• ✅ PTY creation and process spawning
• ✅ Interactive I/O with real terminal emulation
• ✅ Terminal dimensions and resize
• ✅ isatty detection confirms real PTY (not pipes)

Test output:

Testing @replit/ruspty on ARM64 Linux
Architecture: arm64
Platform: linux
----------------------------------------

Test 1: Creating PTY with echo command
  Output: Hello from ARM64!
  ✅ Test 1 PASSED

Test 2: Interactive PTY with bash
  Output: Enter text: You entered: ARM64 Works!
  ✅ Test 2 PASSED

Test 3: Verify real PTY (isatty check)
  Output: isatty: True
  ✅ Test 3 PASSED - Real PTY confirmed!

Test 4: Terminal dimensions
  Output: Cols: 100, Rows: 30
  ✅ Test 4 PASSED

Platform Support Matrix

Platform	Architecture	Status	Binary
Linux	x86_64	✅ Original	ruspty.linux-x64-gnu.node
Linux	ARM64	✅ This PR	ruspty.linux-arm64-gnu.node
macOS	x86_64	✅ Original	ruspty.darwin-x64.node
macOS	ARM64	✅ Original	ruspty.darwin-arm64.node

Build Instructions

# Install Rust toolchain
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

# Build native binary
npm install
npm run build

# Test
node test-arm64.js

Impact

This PR enables ruspty to work on increasingly popular ARM64 Linux platforms without requiring users to build from source. The changes are backward compatible and don't affect existing x86_64 functionality.

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