fix: enable SO_REUSEADDR in native listener for rapid restarts

[oboard]

What

The native Mocket listener previously constructed @http.Server(addr) without reuse_addr, so SO_REUSEADDR was disabled (the pinned moonbitlang/async@0.21.0 TcpServer defaults it to false). During rapid development restarts, a socket left in TIME_WAIT on the same 127.0.0.1:PORT can make the new process report "Address already in use" even though no process is listening.

Fix

Pass reuse_addr=true when building the native HTTP server:

let server = @http.Server(addr, reuse_addr=true) catch { ... }

This lets a fresh process immediately rebind a port a previous process just left in TIME_WAIT.

Also

• Documented the SO_REUSEADDR behavior and the moonbitlang/async BSD/macOS side effect directly on listen_ffi: with reuse enabled, a bind to a specific address may "steal" the interface from an existing wildcard 0.0.0.0 bind, and a wildcard bind can bind to the remaining interfaces if a specific-interface bind already exists.

Verification

• moon check is green on native, js, and wasm.
• Ran the responder example on :4000, sent traffic (HTTP 200), killed the process, and immediately relaunched — the second server successfully rebinds the same port.

Note

The "propagate bind failures as checked errors" improvement is a separate, larger API-signature change (the public Mocket::listen/serve are Unit noraise); I scoped this PR to the primary fix.

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