🐛 fix(singleton): reject conflicting async and lease options

[kokotatan]

Reusing a singleton lock ignored every option a subclass adds on top of the base ones. AsyncFileLock kept the first loop, executor and run_in_executor it got, and SoftFileLease kept the first lease_duration, heartbeat_interval and on_compromise:

from filelock import AsyncFileLock

first = AsyncFileLock("example.lock", is_singleton=True, run_in_executor=False)
second = AsyncFileLock("example.lock", is_singleton=True, run_in_executor=True)
assert first is second
assert second.run_in_executor is False

The documented singleton contract says conflicting parameters raise ValueError. The metaclass sees these options as plain **kwargs, and the subclass alone knows their defaults and how its __init__ resolves them, so each class reports its own mismatches from _singleton_extra_mismatches and merges what super() returns. 🔒 That lets AsyncSoftFileLease check its lease and async options together. Loops, executors and callbacks compare by identity, as on_acquired does, and an omitted heartbeat_interval resolves to a third of the duration before the comparison.

AI assistance: implemented and tested with Codex, then independently reviewed with Codex and Claude Opus.