Implement project-font persistence and defaults in XBuilder

[nighca]

Background

Project-level font support has a design task in #3332 and is tracked by #3342.

XBuilder needs the project-data foundation before editor previews can consume project fonts: projects must own font resources, carry fallback preferences, and preserve those resources through normal project file operations. Editor-side image rendering is tracked separately in #3366.

Goal

Implement the project-font persistence, default-project, and compatibility foundation in XBuilder based on the product model defined in #3332.

Scope

• Add project data structures for font resources and font preferences.
• Persist, publish, export, and load project font files through existing project file mechanisms.
• Serialize fontPreferences in project configuration and expose the parsed data through the project model.
• Configure deployment-specific new-project font preferences and materialize required preset font files.
• Preserve the existing Chinese rendering behavior when legacy projects are opened and subsequently saved.
• Assemble the default project from lazily loaded source assets through project-model APIs rather than an .xbp template.
• Remove Builder-bundled Scratch font files while retaining the generic SVG font-injection utility for the editor follow-up.

Out of scope

• Editor-side rendering of project fonts in SVG/image previews; tracked by Render project fonts in editor #3366.
• Final product UI for adding/removing project fonts or changing preferences.
• Expanding the base SPX runtime by baking in every possible font.
• Depending on fonts installed on the user's local system for published/exported project behavior.

Related

• Parent tracking issue: Track project-level font support #3342
• Product design issue: Add project-level font support design #3332
• Editor rendering follow-up: Render project fonts in editor #3366