…ntinel

When resolving an async factory with 2+ async dependencies, the compiled
getter now awaits them concurrently via asyncio.gather instead of
sequentially. For N independent deps each taking T seconds, resolution
takes max(T) instead of sum(T). No user-side changes are required.

This is implemented as a compile-time code generation change: the
compiled factory function is generated with asyncio.gather instead of
sequential inline awaits. The cache dict semantics and CompiledFactory
protocol are unchanged, and the sync container is unaffected.

Diamond patterns (A→C, B→C) are handled via a _Pending Future sentinel:
the first coroutine to resolve a cached dep places a sentinel in the
cache; concurrent coroutines await it instead of duplicating work and
registering duplicate exits. Uncached (cache=False) shared deps are also
safe to gather since each branch creates its own independent instance.

reagento#45