WASI 0.3 adds native async I/O via future<T> and stream<T> types (stable in wasmtime 37+, November 2025). This is the prerequisite for true concurrent request handling in Vera's server-side effects.

Depends on WASI 0.2 compliance (#237).

Scope

• Migrate from the WASI 0.2 incoming-handler model to WASI 0.3 async I/O
• Wire native future<T>/stream<T> to Vera's <Async> effect (currently eager/sequential)
• Enable concurrent request handling in <HttpServer> (<HttpServer> effect — verified HTTP request handling #305) and <McpServer> (<McpServer> effect — verified MCP tool server #306)
• Support WebSocket/SSE streaming (WebSocket and SSE client effects #228)