Introduces internal/requestid package with shared context key to avoid
circular imports between server and middleware packages. Server's
RequestID middleware now uses the shared key. Client middleware picks up
the ID from context and sets X-Request-Id on outgoing requests.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Introduces server/ sub-package as the server-side companion to the existing Client.
Includes Router (over http.ServeMux with groups and mounting), graceful shutdown with
signal handling, health endpoints (/healthz, /readyz), and built-in middlewares
(RequestID, Recovery, Logging). Zero external dependencies.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
