Report a clear error and exit cleanly when the HTTP server can't bind its port

[alistair3149]

Summary

app.listen emits its 'error' event asynchronously — after startHttpServer() has already returned — so index.ts's main().catch never sees it. With no 'error' listener, a bind failure (EADDRINUSE, EACCES) surfaced as an uncaught exception with a raw stack trace.

This attaches an error handler so a bind failure produces a clear, actionable message and a clean exit(1):

Cannot start HTTP server: 127.0.0.1:3000 is already in use.

What changed

• New handleListenError(err, host, port, onFatal?) in streamableHttp.ts: logs EADDRINUSE / EACCES / generic distinctly and terminates. onFatal is injectable so it is unit-testable without exiting the test process. Wired via httpServer.on('error', …) in startHttpServer().
• Guard the success log: Express fires the app.listen callback even when the bind failed (httpServer.listening is false then), which would otherwise print a misleading listening on … line right before the failure. A bind failure now logs only the clear error.

This is the deferred follow-up from the boot-extraction (#466), now unblocked since startHttpServer() is on master.

Testing

• Test-driven unit tests for handleListenError (EADDRINUSE / EACCES / generic → correct message + onFatal(1)).
• Real two-servers-on-one-port smoke: the second process exits 1, logs only Cannot start HTTP server: … is already in use., and prints no stack trace and no misleading listening line; a successfully-bound server still logs listening.
• Full gate (fmt:check, lint, tsc, tests) green.

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