Add safe project asset management and publish Beta 7

[WebCrew]

Summary

• add selectable project assets with a focused Rename / Move workflow
• validate all asset paths inside assets/, reject symbolic-link escapes and preserve file extensions
• use non-overwriting filesystem operations and create safe nested asset folders
• add dedicated automated asset contract coverage and preload-boundary checks
• document the Phase 2 milestone and prepare the unsigned Beta 7 test release

Scope

This intentionally does not add Phase 4 importer metadata, stable asset IDs, reimport behavior or reference migration. The current scene format does not yet store live asset references.

Verification

• node scripts/check.cjs
• JavaScript syntax checks for main, renderer and asset modules
• git diff --check

The local clean renderer build could not run because the workspace has no installed esbuild binary. GitHub CI installs the locked dependencies before performing the authoritative renderer and Windows build.