chore: update MoonBit toolchain compatibility

[brickfrog]

Summary

• migrate module and executable manifests to the current MoonBit syntax
• make native the preferred development target and align contributor instructions
• replace deprecated Bytes::default() calls with empty byte literals
• report the complete MoonBit toolchain identity in CI

Why

The floating MoonBit installer now selects a newer composite toolchain. Without
this cleanup, bare moon test selects the unsupported Wasm1 async path and
moon fmt rewrites tracked manifests, causing CI's format check to fail. The
new core snapshot also deprecates the promoted Bytes::default() form.

This keeps existing behavior and public interfaces unchanged while making the
repository compatible with the current toolchain.

Validation

• moon info (no .mbti changes)
• moon fmt (idempotent)
• bare moon test — 279 passed
• moon check --target native
• moon check --target js
• moon check --target wasm-gc
• moon test --target native — 279 passed
• moon test --target js — 270 passed
• moon test --target wasm-gc — 255 passed
• moon build --target native --release
• git diff --check

No version bump or release is needed because this does not change the published
API or runtime behavior.