P0-S1c: Implement io::poll (always-ready) for streams

[mconcat]

P0-S1c: Implement io::poll (always-ready) for streams

Background

WASI Preview 2 streams rely on io::poll for readiness. For P0, we provide always-ready semantics because our streams operate over in-memory buffers/overlay.

Goal

Implement the minimal io::poll host wiring to report readiness for our VfsInputStream/VfsOutputStream.

Design

• Add host-side io::poll support returning ready for registered stream handles.
• Ensure binding functions compile and contracts using standard I/O don’t block.

Constraints

• P0 only: no async backends, hence always-ready acceptable.

Acceptance Criteria

• Integration usage of streams does not require blocking or async polling.
• Unit tests confirm poll returns ready for both stream types.

References

• Epic: P0: WASI File I/O Streams over VFS with Merkle-backed Persistence #79; S1a: P0-S1a: Implement VfsInputStream & VfsOutputStream (wasmtime-wasi-io) #80; S1b: P0-S1b: Wire read_via_stream/write_via_stream/append_via_stream #81; PR: Feat/vfs #78