docs: add project plan and development rules

- docs/plan.md: project overview, architecture, milestones, directory structure
- DEVELOP.md: dev rules, code quality standards, git workflow, testing rules, milestone tracking

DEVELOP.md

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+# DEVELOP.md — Development Rules & Tracking
+
+## Quick Start
+
+```bash
+# Prerequisites
+rustup show            # Rust 1.80+
+cargo install cargo-watch
+
+# Run locally
+cargo run -- --config config/default.toml
+
+# Run with auto-reload
+cargo watch -x run
+
+# Run tests
+cargo test
+
+# Lint
+cargo clippy -- -D warnings
+cargo fmt --check
+```
+
+## Development Rules
+
+### Code Quality
+
+1. **No warnings.** All code must compile with zero warnings. CI treats warnings as errors (`-D warnings`).
+2. **Format before commit.** Run `cargo fmt` — CI checks formatting.
+3. **Clippy clean.** Run `cargo clippy -- -D warnings` — CI checks lints.
+4. **Tests required.** Every new module/function must have unit tests. Public API changes require integration tests.
+5. **Doc comments.** All public items (`pub fn`, `pub struct`, `pub enum`) must have `///` doc comments.
+
+### Git Workflow
+
+1. **Branch naming**: `<type>/<short-description>`
+   - `feat/dns-cache`, `fix/ttl-overflow`, `docs/readme`, `chore/ci-update`
+2. **Commit messages**: Conventional Commits format
+   - `feat: add DNS response cache`
+   - `fix: handle truncated UDP responses`
+   - `docs: update plan.md`
+3. **PR required.** All changes go through a pull request. No direct pushes to `main`.
+4. **Squash merge.** PRs are squash-merged into `main`.
+5. **CI must pass.** Lint, test, and build must succeed before merge.
+
+### Architecture Rules
+
+1. **No panics in production code.** Use `Result<T, E>` everywhere. `unwrap()` only in tests.
+2. **Structured errors.** Define error types per module using `thiserror`. Use `anyhow` only in `main.rs`.
+3. **Async everywhere.** All I/O is async (Tokio). No blocking calls in async context.
+4. **Config-driven.** No hardcoded values. All tunables come from config file or env vars.
+5. **Dependency injection.** Accept traits, not concrete types. Makes testing easy.
+
+### Testing Rules
+
+1. **Unit tests** in the same file (`#[cfg(test)] mod tests { ... }`).
+2. **Integration tests** in `tests/` directory.
+3. **Test naming**: `test_<what>_<condition>_<expected>` — e.g., `test_cache_expired_entry_returns_miss`.
+4. **No external dependencies in unit tests.** Mock upstream resolvers.
+5. **Integration tests** may spin up the full server on a random port.
+
+### Security Rules
+
+1. **No secrets in code.** Use env vars or config files (gitignored).
+2. **Validate all input.** DNS queries from the network are untrusted — validate lengths, offsets, labels.
+3. **Timeouts on all upstream calls.** Never hang indefinitely.
+4. **Rate limiting.** Protect against abuse from a single IP.
+5. **Dependency audit.** Run `cargo audit` in CI.
+
+### Commit Checklist
+
+Before pushing, verify:
+
+- [ ] `cargo fmt --check` passes
+- [ ] `cargo clippy -- -D warnings` passes
+- [ ] `cargo test` passes
+- [ ] New code has tests
+- [ ] Public items have doc comments
+- [ ] No `unwrap()` outside of tests
+- [ ] Config values are externalized (not hardcoded)
+
+## Milestone Tracking
+
+See [docs/plan.md](docs/plan.md) for the full project plan and milestone checklist.
+
+| Milestone | Status |
+|-----------|--------|
+| M1 — Core Forwarding | 🔲 Not started |
+| M2 — Multiple Upstreams & Config | 🔲 Not started |
+| M3 — Caching | 🔲 Not started |
+| M4 — Observability | 🔲 Not started |
+| M5 — Blocking & Filtering | 🔲 Not started |
+| M6 — Production Hardening | 🔲 Not started |
+
+## Useful Commands
+
+```bash
+# Build release binary
+cargo build --release
+
+# Run with specific log level
+RUST_LOG=debug cargo run
+
+# Generate and open docs
+cargo doc --open
+
+# Dependency audit
+cargo install cargo-audit
+cargo audit
+
+# Watch tests
+cargo watch -x test
+```