P0-S1a: Implement VfsInputStream & VfsOutputStream (wasmtime-wasi-io)

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P0-S1a: Implement VfsInputStream & VfsOutputStream (wasmtime-wasi-io)

Background

We are integrating a Virtual Filesystem (VFS) with WASI Preview 2 so contracts perform std::fs operations that persist to a Merkle KV store. Current state (per internal status and PR #78):

• Our host exports wasi:filesystem/* and non-FS subsystems; components instantiate successfully.
• Descriptors, preopens, open_at (files/dirs), directory iteration, mkdir/unlink/rename/rmdir(empty), and set_size are implemented.
• Streams (wasi:io/streams, io:poll) are not implemented; read_via_stream/write_via_stream return Unsupported.

This sub-issue delivers the concrete stream types to back file I/O over the VFS.

Goal

Implement VFS-backed InputStream and OutputStream types that satisfy wasmtime-wasi-io traits and operate on our file descriptors, enabling buffered read/write semantics with an internal offset.

Design

• Define host-side types:
  • VfsInputStream { fd: u32, offset: u64, ctx: Weak<VfsCtx> }
  • VfsOutputStream { fd: u32, offset: u64, ctx: Weak<VfsCtx>, append: bool }
• Implement wasmtime_wasi_io::streams::InputStream and OutputStream:
  • Input: read(size) -> Bytes, copy from file contents at offset, advance offset; EOF => empty bytes.
  • Output: write(bytes), apply to the per-execution overlay at offset (or end if append), advance offset. flush() no-op for P0.
• Error mapping: surface I/O errors as StreamError; descriptor validation errors bubble via the binding call sites.
• Determinism: do not read external time/random; deterministic overlay updates only.

Constraints

• WASI Preview 2, wasmtime 34.x.
• Do not fork wasmtime-wasi; use public wasmtime-wasi-io traits.
• These objects will later be boxed into the resource table by binding fns (see S1b).

Acceptance Criteria

• Stream types compile and can be constructed in tests with a stubbed context.
• Input returns bytes and empty on EOF; Output writes bytes and advances offset.
• Unit tests cover offset behavior and basic error flows.

Non-Goals (P0)

• Poll/readiness beyond always-ready (handled in S1c).
• Direct preview2 read/write syscalls (we keep them Unsupported).

References

• Epic: P0: WASI File I/O Streams over VFS with Merkle-backed Persistence #79
• Streams wiring: P0-S1b: Wire read_via_stream/write_via_stream/append_via_stream #81
• Poll integration: P0-S1c: Implement io::poll (always-ready) for streams #82
• Stat-by-fd: P0-S2a: Stat-by-fd helper and accurate sizes #83
• Backend overlay/commit: P0-S2b: Merkle adapter + transaction overlay and commit #84
• PR: Feat/vfs #78