enhancement: WASI 0.3 async support (streams, futures, event loop)

Tracked by #92

## Goal

Add WASI 0.3 async support: `async` functions, `stream`, `future`, and a host-managed shared event loop. Optionally polyfill 0.2 on top of 0.3.

## Status

Scope to be refined after #92 Phase 3 (0.2 worlds) lands. WASI 0.3 was ratified 2026-06-11; keep opt-in until language toolchains mature.

## Depends on

- Phase 3 (WASI 0.2 worlds)

Start by reading issue #92 and its Phase 3 work on WASI 0.2 worlds, which this issue depends on. After that scope is refined, the target is opt-in WASI 0.3 support for async functions, streams, futures, and a host-managed shared event loop; an optional 0.2 polyfill is also mentioned.

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