Create CLAUDE.md

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Stackforge is a high-performance networking stack and automata framework written in Rust with Python bindings via PyO3/maturin. It provides Scapy-like packet manipulation with native Rust performance.
+
+## Build & Development Commands
+
+```bash
+# Install dependencies and set up virtual environment
+uv sync
+
+# Build Rust extension and install in editable mode (required after Rust changes)
+uv run maturin develop
+
+# Run all Rust tests
+cargo test
+
+# Run a specific Rust test
+cargo test test_name
+
+# Run Python tests
+uv run pytest tests/python
+
+# Lint and format
+cargo fmt
+cargo clippy
+uv run ruff check .
+```
+
+## Architecture
+
+### Crate Structure
+
+- **`stackforge` (root)** - PyO3 bindings that expose Rust functionality to Python. Compiles to a `cdylib`.
+- **`stackforge-core`** - Core networking primitives implementing the "Lazy Zero-Copy View" architecture.
+- **`stackforge-automata`** - Async state machine framework for network automation (placeholder, planned for future).
+
+### Lazy Zero-Copy View Architecture
+
+The core design pattern in `stackforge-core`:
+
+1. **Zero-Copy Buffers**: Packets stored as `bytes::Bytes` (reference-counted, clone-on-write)
+2. **Index-Only Parsing**: `Packet::parse()` only identifies layer boundaries (start/end offsets), not field values
+3. **On-Demand Field Access**: Fields read directly from buffer when accessed (e.g., `eth.ethertype(buf)`)
+4. **Copy-on-Write Mutation**: `Packet::with_data_mut()` clones buffer only when shared
+
+### Key Types
+
+- `Packet` (`packet.rs`) - Main packet container with `Bytes` buffer and `SmallVec<LayerIndex>`
+- `LayerKind` - Enum identifying protocol types (Ethernet, ARP, IPv4, TCP, etc.)
+- `LayerIndex` - Lightweight struct storing `kind`, `start`, `end` offsets
+- `Layer` trait - Interface for protocol layers with `summary()`, `header_len()`, `hashret()`, `answers()`
+- Protocol layers (`EthernetLayer`, `ArpLayer`, `Ipv4Layer`, `TcpLayer`) - Views into packet buffer
+
+### Layer Module Organization
+
+`crates/stackforge-core/src/layer/`:
+- `mod.rs` - `LayerKind`, `LayerIndex`, `Layer` trait, `LayerEnum` dispatch
+- `field.rs` - Field abstraction with `MacAddress`, `BytesField`, `FieldValue`
+- `bindings.rs` - Protocol relationships (e.g., Ethernet type 0x0806 → ARP)
+- `ethernet.rs`, `arp.rs`, `ipv4/`, `tcp/` - Protocol-specific implementations
+- `neighbor.rs` - ARP/NDP cache implementations
+
+### Python Bindings
+
+- `src/lib.rs` - PyO3 module exposing `Packet`, `LayerKind`, `LayerIndex`
+- `python/stackforge/__init__.py` - Re-exports from compiled extension
+- Python source configured via `tool.maturin.python-source = "python"` in pyproject.toml
+
+## Testing
+
+- Rust unit tests: inline in source files and `crates/stackforge-core/src/lib.rs`
+- Rust integration tests: `tests/integration/`
+- Python tests: `tests/python/test_core.py`