dev derives additional ports from --port + offsets (metrics, gRPC,
debug). A --port near uint16 max would overflow silently; add up-front
validation that rejects values that would push derived ports over 65535.
Updates #3157
Add a lightweight dev tool that starts a headscale server on localhost
with a pre-created user and pre-auth key, ready for connecting real
tailscale nodes via mts.
The tool builds the headscale binary, writes a minimal dev config
(SQLite, public DERP, debug logging), starts the server as a
subprocess, and prints a banner with the server URL, auth key, and
mts usage instructions.
Usage: go run ./cmd/dev
make dev-server