Explicitly target WASI 0.2 compliance, with a migration path to WASI 0.3 for async.

Motivation

WASI standardizes the interface between WASM modules and host environments. Explicit compliance ensures Vera programs are portable across runtimes (wasmtime, wasmer, wazero). WASI 0.3 (release candidate in Wasmtime 37+, November 2025) adds native async with stream<T> and future<T> types that align with Vera's planned Async effect.

Current state

Vera currently uses ad-hoc host function imports for IO operations. These work with wasmtime but are not WASI-compliant.

Proposed work

• Align existing IO operations (print, read_line, read_file, write_file, args, exit, get_env) with WASI 0.2 interfaces
• Document WASI 0.3 as the target for the Async effect (Async futures and promises (<Async>) #59)
• Ensure compiled WASM modules declare WASI imports correctly

Dependencies

• Async futures and promises (<Async>) #59 (Async) — WASI 0.3 alignment for async