Add dynamic worker pool with bounded queue and concurrency tests

README.md

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+# worker
+
+`worker` is a small Go pool for `func()` tasks. Create it with a minimum and
+maximum worker count, then submit tasks:
+
+```go
+pool, err := worker.New(2, 8)
+if err != nil {
+	return err
+}
+defer pool.Close()
+
+if err := pool.Submit(func() { doWork() }); err != nil {
+	return err
+}
+```
+
+The pool creates the minimum workers immediately and grows on demand up to the
+maximum. Its task queue has a fixed capacity of `maxWorkers/2`; once all workers
+and queue slots are occupied, `Submit` waits for room. `Close` stops new
+submissions and waits for accepted tasks to finish. A successful `Submit` means
+the task was accepted; it does not mean the task has started or finished. With
+`maxWorkers == 1`, the queue has zero capacity and submissions wait for a worker.
+
+After a task finishes, a worker exits immediately only when `total > min` and
+the number of *other* idle workers is greater than `min/2`. There is no idle
+timeout. A task panic is logged and that worker continues with later tasks.
+
+Tasks should return normally or panic. Do not call `runtime.Goexit` from a task:
+it terminates the worker and may leave `Close` waiting indefinitely. Call
+`Close` from outside pool tasks so it does not wait for itself.
+
+A task may submit another task to the same pool. If all workers block submitting
+to a full queue, the caller must resolve that deadlock in its task logic.