fix: reject unsupported wasm targets at the package boundary

[oboard]

Problem

At main@c545db2, the README states Mocket supports the JS and native backends, but moon.pkg still selects mocket.wasm.mbt and serve.mbt for wasm/wasm-gc. As a result, server code using Mocket::listen type-checks for those targets and then traps at runtime:

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

Fix

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.

The cors and static_file sub-packages depend on oboard/mocket and are likewise js/native-only, so they now declare supported_targets explicitly (this also removes the "does not declare supported_targets" warnings they would otherwise emit once the parent package declares its targets).

Verification

• moon check --target wasm on a wasm consumer that calls Mocket::listen now fails at build time:

  Selected backend 'wasm' is incompatible with the dependency graph. '...' requires 'oboard/mocket' which supports [js, native].
  

• moon check --target wasm-gc fails the same way.

• moon check --target js and --target native still pass.

• moon check --target all finishes cleanly with no warnings.

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