Migrate deprecated MoonBit syntax and normalize manifests

[Milky2018]

Summary

• replace deprecated type-parameter method promotion with explicit defining-trait dispatch
• migrate executable package declarations to pkgtype(kind: "executable")
• refresh formatter output and generated package interfaces
• record the pre-existing deferred Commands::spawn regression as ISS-474

Why

The updated MoonBit toolchain reports warning 83 for legacy method-call syntax on type-parameter receivers. The formatter and interface generator also normalize executable manifests and generated interfaces for the current toolchain.

No runtime behavior or intentional public API change is expected.

Validation

• MOONBIT_NEW_NATIVE=1 moon -C mgstudio-engine check --target native --deny-warn
• MOONBIT_NEW_NATIVE=1 moon -C mgstudio-engine build --target native --release --deny-warn examples/stress_tests/many_foxes
• moon fmt
• moon info

Aggregate tests were intentionally not rerun. The per-package focused-test criterion remains open, and the unrelated Commands::spawn abort is tracked in ISS-474. git diff --check reports only three generated pkg.generated.mbti EOF blank lines from moon info.