refactor(components): migrate ZoneComponent and ScreenComponent to class fields

[willeastcott]

Summary

Migrates the constructor-initialised instance state of ZoneComponent and ScreenComponent to ES2022 public class field declarations, continuing the modernisation pass applied earlier to component data and several other components.

• ZoneComponent: moves _oldState and _size out of the constructor; the set_enabled listener wiring stays in the constructor.
• ScreenComponent: moves 10 instance fields out of the constructor. The graphicsDevice.on('resizecanvas', ...) wiring stays in the constructor. One minor simplification: this.cull = this._screenSpace; becomes cull = false; — the original value was always false at that point (_screenSpace had just been assigned false on the line above), so the default is unchanged.
• /** @private */ tags added to the newly-declared underscore-prefixed fields to match the convention already used elsewhere in the codebase (e.g. button/component.js, layout-child/component.js).

Pure internal refactor. No behavioral change, no public API change.

Test plan

• npm run lint — clean.
• npm test — 1672 tests passing.

[Copilot]

Pull request overview

This PR continues the component modernisation work by moving constructor-initialized instance state for ZoneComponent and ScreenComponent into ES2022 class field declarations. In the framework, that keeps component defaults closer to their declarations while leaving runtime wiring in the constructors.

Changes:

• Moved ZoneComponent’s _oldState and _size defaults from the constructor to class fields.
• Moved ScreenComponent’s per-instance defaults (Vec2, Mat4, Set, scalar flags, and cull) from the constructor to class fields.
• Kept constructor side effects intact, including set_enabled binding for zones and resizecanvas subscription for screens.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/framework/components/zone/component.js	Declares zone private instance state as class fields instead of assigning it in the constructor.
src/framework/components/screen/component.js	Declares screen default instance state as class fields and preserves constructor event wiring.

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