fix(core): robust dev server — startup port reclaim, singleton guard, :3000 EADDRINUSE handling (#136)

Co-authored-by: H. Furkan Bozkurt <bozkurh@amazon.com>
Co-authored-by: H. Furkan Bozkurt <hfurkanbozkurt@users.noreply.github.com>

Author: hfurkanbozkurt

.changeset/dev-server-robustness.md

@@ -0,0 +1,26 @@
+---
+"@aws-blocks/core": patch
+---
+
+fix(core): robust dev server — startup port reclaim, singleton guard, and :3000 EADDRINUSE handling
+
+Hardens the local dev server against the port-contention failure modes that
+survived PR #80 (which fixed only single-supervisor frontend self-restart):
+
+- **Startup reclaim** — a fresh `server.ts` now frees a stale `:3000`/`:3100`
+  listener left by a crashed or `SIGKILL`'d predecessor before it binds the
+  front door / spawns the `--strictPort` frontend, instead of relying on the
+  previous process's `cleanup()` finishing within tsx-watch's ~5s window.
+- **Singleton guard** — a per-port pidfile stops a second `npm run dev` from
+  spawning a competing supervisor that fights the first over `:3000`/`:3100`; it
+  exits cleanly with a clear message. A stable-parent (`tsx watch`) carve-out
+  keeps hot reload working, and a dead-owner pidfile never blocks startup.
+- **`:3000` EADDRINUSE robustness** — the backend front door now emits a real
+  console error (not only telemetry), reclaims the stale owner and retries the
+  bind (bounded), and exits non-zero with a clear message on unrecoverable
+  failure, so a contended `:3000` never silently fails to serve.
+
+Reuses the existing `waitForPortFree` / process-group-kill primitives (plus an
+`lsof`/`netstat` listener probe mirroring the `cleanup` script) — no new
+teardown mechanism. `--strictPort` is retained for local dev, made safe by the
+startup reclaim guaranteeing `:3100` is free first.