feat: Create blank text assets

[kof]

Summary

• add a Create text file action to the Assets toolbar and shared context/dropdown menu
• validate filenames against the central editable-asset registry, create the empty file in the current asset folder, and open it in the text editor
• keep simultaneous empty uploads distinct by including the filename in the upload fingerprint
• keep internal Nano ID storage suffixes out of visible asset names and subsequent content revisions
• show the immutable file extension beside the editable basename in Asset settings

Asset querying, content hydration, structured data, bindings, generated runtime support, and MCP integration remain owned by #5921.

Verification

• Builder asset/text tests: 34 passed
• SDK asset tests: 110 passed
• project-build asset tests: 71 passed
• asset-uploader revision tests: 5 passed
• SDK, project-build, asset-uploader, and Builder typechecks
• lint and package-boundary checks

Ref #5824