feat: add wasip3 behind flag

[ricochet]

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[ricochet]

I'm going to close this in favor of someone else having the opportunity to start fresh on finding the right abstraction. The actual work is figuring out where the seams should be. Some concrete questions to investigate:

• What does AsyncRead/AsyncWrite look like as a single trait that both p2 streams and p3 streams implement?
• Can block_on and the reactor be unified? p3 doesn't need a reactor, but the public API of wstd::runtime could be the same, perhaps a no-op shim under p3?
• The #[http_server] macro is the hardest case because the WASI export traits are genuinely different. Possible approaches: a sealed HttpServer trait that both versions implement, with the macro generating the impl.
• Today the diff has BodyInner::Incoming and BodyInner::P3Stream as parallel variants. A single internal trait BodyBackend with two implementations would be better.

Any version-specific code should be at the bottom of the abstraction stack, not woven through.