Add CLAUDE.md with build commands and architecture notes

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kmrgkpp9YBJ6AMaWYPPZuD

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.
+
+## What this is
+
+A minimal Rust reimplementation of the Cyrus `saslauthd` daemon. It listens on a Unix
+socket, speaks the saslauthd wire protocol, and checks credentials against per-user
+password files in a directory. In production it runs as PID 1 in an Alpine container
+(`pobox_saslauthd`), and Cyrus IMAP / Exim authenticate through its socket.
+
+## Commands
+
+```sh
+cargo build --workspace
+cargo test --workspace                          # unit + integration tests
+cargo test -p server --test shutdown_test       # one integration test file
+cargo test -p server it_checks_argon2           # one test by name
+cargo clippy --workspace --all-targets          # plain `cargo clippy` skips test code
+cargo run -p server -- -f /tmp/saslauthd.sock -p ./test_passwords
+RUST_LOG=info cargo run -p server               # env_logger; info shows auth results + shutdown
+cargo run -p client -- -f /tmp/saslauthd.sock bp some-rad-password   # manual round-trip
+docker build -t saslauthd .                     # musl static build, targets x86_64 only
+```
+
+Tests spawn the real `saslauthd` binary on a socket under the OS temp dir and signal it
+with `kill`, so they are Unix-only and rely on `test_passwords/bp` at the repo root.
+
+## Architecture
+
+Cargo workspace (edition 2024) with three crates:
+
+- `common` — the wire protocol shared by server and client. `io.rs` reads/writes
+  length-prefixed strings (u16 big-endian length + bytes). `request.rs` is the four-field
+  request (userid, password, service, realm) in that order.
+- `server` — the `saslauthd` binary. `Server` owns the `UnixListener`, spawns one thread
+  per connection running `Handler`, which parses a `Request`, asks `PasswordDirectory`
+  whether it is valid, and writes back `OK` or `NO` as a length-prefixed string.
+- `client` — `saslauthd_test_client`, a tiny CLI for exercising a running server.
+
+Top-level `tests/` holds the `common` crate's protocol tests; `server/tests/` holds the
+process-level shutdown tests.
+
+### Authentication model (`server/src/repository.rs`)
+
+The password directory contains one file per userid. Blank lines and `#` comments are
+ignored. Each remaining line is either:
+
+- a plain-text password, matched exactly, or
+- `[key]<argon2 PHC hash>`, matched only when the client sends the password as
+  `[key]<plaintext>` and the plaintext verifies against the hash for that key.
+
+The client's password prefix decides which path is taken. Userids containing `/` are
+rejected before touching the filesystem. `service` and `realm` are read but ignored.
+
+### Server lifecycle (`server/src/server.rs`)
+
+Startup removes any stale file at the socket path, binds, and chmods the socket 0777.
+The accept loop is non-blocking and polls a flag set by `signal-hook` for SIGTERM/SIGINT.
+On a signal it stops accepting, waits up to 3 s for in-flight handler threads, then
+`Drop` removes the socket file. Exit status is 0. This matters because as container
+PID 1 the process gets no default signal handling; without it `docker stop` hangs for
+10 s and host reboots stall.
+
+CLI options live in `options.rs` as a `LazyLock<Opt>` (clap derive) and are read from
+anywhere via `OPTIONS`.
+
+## Constraints
+
+- Do not change the wire protocol or the `--socket-name` / `--password-dir` flags;
+  Cyrus and Exim depend on both.
+- The Dockerfile hardcodes `x86_64-unknown-linux-musl`. On an arm64 Mac the cross-build
+  segfaults under qemu; verify container behaviour by substituting
+  `aarch64-unknown-linux-musl` in a copy of the Dockerfile instead.