[wasm] Mark server APIs unsupported instead of panicking at runtime

[yorkin-bot]

Summary

At main@c545db2 (module version 0.8.0), the README correctly says that Mocket supports the JS and native backends. However, the package configuration still selects mocket.wasm.mbt and serve.mbt for wasm/wasm-gc, so server code using Mocket::listen type-checks successfully for those targets and then traps at runtime.

Relevant code:

• moon.pkg selects the Wasm implementation
• serve_ffi and listen_ffi are panic stubs

Reproduction

fn main {
  @mocket.new().listen("127.0.0.1:49401")
}

With an executable package that declares Wasm support:

moon check <package> --target wasm
# succeeds

moon check <package> --target wasm-gc
# succeeds

moon run <package> --target wasm
# RuntimeError: unreachable
#   at @oboard/mocket.listen_ffi
#   at @oboard/mocket.Mocket::listen

Expected behavior

Unsupported server targets should be rejected at build/check time rather than exposing public APIs that always panic.

The direct fix is to declare the package's actual backend support at the top level of moon.pkg:

supported_targets = "js+native"

This makes moon check/moon build reject wasm and wasm-gc consumers at the package boundary, matching the README and avoiding runtime-only panic stubs. If a functional Wasm/WASI server backend is implemented later, the target set can be expanded then.

[oboard]

#23 #24