feat(buffer): async + collapsing buffer for awaitable batching

[shavounet]

Stale work — opening as draft to keep it visible. Needs tests + polish before it can be considered for merge.

Summary

• Adds AsyncBuffer: register args, get a Promise back, the buffer auto-flushes on
  event-loop idle, on min/max wait time, or on size threshold. Caller-supplied flusher
  generator resolves the deferred items.
• Adds CollapsingBuffer: thin layer over AsyncBuffer that deduplicates by key —
  duplicate registrations return the same in-flight Promise.
• Adds UnresolvedItemsException thrown when the flusher leaves items pending.
• Adds psr/log: ^3.0 for optional debug logging inside AsyncBuffer.

Follow-ups before merge

• Tests for AsyncBuffer (idle/min-wait/max-wait/size triggers, exception path)
• Tests for CollapsingBuffer key dedup
• Add declare(strict_types=1) to CollapsingBuffer.php and UnresolvedItemsException.php
• Type UnresolvedItemsException::getItems(): array and tighten the array<> generics