fix: detect port-in-use before starting (#1234)

[echopi]

Summary

• Add server.on('error') handler in lib/index.js to catch EADDRINUSE and print a friendly error message
• Use exclusive: true in server.listen() to ensure port conflicts are detected across platforms (macOS uses SO_REUSEADDR by default)
• Add standalone port-in-use test case

Fixes #1234

What changed

lib/index.js (+10/-1):

1. Error handler on main HTTP server — prints friendly message on EADDRINUSE:

   [!] Port 8899 is already in use.
   [i] Use -p <port> to specify another port.
   

2. server.listen() changed to use { exclusive: true } to prevent SO_REUSEADDR from masking port conflicts

test/units/port-in-use.test.js + test/helpers/start-whistle.js: Standalone test that occupies a port, starts whistle on the same port, and verifies the friendly error message appears.

Design decisions

Concern	Decision
Where to handle	lib/index.js server error event, not index.js pre-check — avoids TOCTOU race, preserves sync return value
exclusive: true	Required on macOS where SO_REUSEADDR allows duplicate binds silently
Error re-thrown after message	Lets existing uncaughtException handler in same file do process cleanup

Test plan

# Standalone port-in-use test
node test/units/port-in-use.test.js

• Port occupied → friendly error message, non-zero exit
• Verified on macOS with exclusive: true

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