docs: add README, LICENSE files, and crate/module-level doc comments

Add top-level README.md with alpha status warning, quick start guide,
architecture overview, feature flags, transport modes, auth docs, and
Node.js API examples.

Add dual LICENSE-MIT and LICENSE-APACHE files.

Add comprehensive crate-level and module-level rustdoc to all three
crates (wraith-core, wraith, wraith-napi) and all public modules
(transport, client, server, auth, socks5, error). Add doc comments to
key public types (Transport, TransportAcceptor, ConnectOptions,
ClientSession, Server, ServeOptions, KeySource, ServerAuthConfig, etc).

Update Cargo.toml files with workspace-level package metadata
(version, edition, license, repository) and crate descriptions.

crates/wraith-napi/src/serve.rs

@@ -1,3 +1,9 @@
+//! NAPI `serve()` function and `WraithServer` type.
+//!
+//! Starts a TCP-based SSH server that emits new channel streams via a
+//! `ThreadsafeFunction` callback. Currently supports TCP transport only;
+//! TLS and iroh will be added in a future release.
+
 use std::net::SocketAddr;
 use std::sync::Arc;