perf(body): simpler custom Incoming channel

[seanmonstar]

The previous channel variant for Incoming used several individual channels to handle the various mechanisms: sending data, signaling error/abort, sending trailers, waiting for optional expect-101 polling. Each individual channel would allocate its own internal shared state in an Arc.

Now the Incoming channel variant uses a single combined implementation, putting all the required shared pieces in a single Arc. This resulted in less allocations, and less places to potentially store wakers.

The following performance improvements were noticed with microbenchmarks:

Benchmark	Before	After	Improvement
Create/drop	400 ns	130–139 ns	~66%
Data handoff	229 ns	147–155 ns	~34%
Expect-101 wait	492 ns	267–298 ns	~40–46%

• The description, docs, and comments (words meant for humans) are written by a human, not by an LLM. (https://seanmonstar.com/micro/20260729-i-want-your-own-words/)