Implement a Thread-Safe Per-Key Ordered Executor

Implement an in-process component constructed with maxConcurrent and an existing thread-pool executor. It exposes submit(key, task) and must preserve task order within each key while allowing independent keys to execute concurrently.

Constraints & Assumptions

• Multiple threads may call submit concurrently. Define the linearization point that establishes order for concurrent submissions sharing a key.
• Tasks with one key must execute in submission order and must never overlap.
• Tasks with different keys may run concurrently, with at most maxConcurrent tasks running globally.
• submit returns after enqueueing/scheduling; it does not wait for the task to finish.
• Do not implement the underlying thread pool. For the core algorithm, assume it accepts scheduled work while this component is open and that tasks eventually return or throw.
• A long task for one key must not hold a lock or occupy extra waiting workers that unnecessarily block other keys when capacity is available.

Clarifying Questions to Ask

• Should queue capacity be bounded, and how should overload be reported?
• What should task exceptions do to later tasks for the same key?
• Is shutdown or executor rejection part of the required interface?