refactor(asset): unify subclass fields via parameter properties

[krdlab]

Summary

Unify how HaxeAsset and NekoAsset declare their constructor-derived fields. The nightly field, previously declared as nightly = false; and assigned in the constructor body, is now a protected readonly parameter property — matching the existing force32 style on NekoAsset. No behavior change.

• Drops the redundant default initializer and this.nightly = nightly assignment.
• Tightens visibility from implicit public to protected readonly (no external readers exist).
• Regenerates dist/index.js to keep the bundle in sync (CI enforces this).

Test plan

• npm run build (tsc) passes
• npm test passes (58/58 in asset.test.ts)
• dist/ sync check passes (re-bundled via npm run dist)