fix: a system file can't stop the server

[over2take]

A formula calling constructor(1) or toString(1) passed every check, since the built-in function table was a plain object, and then threw on each save of a sheet in that system, inside database callbacks where nothing caught it. The table has no prototype now. A buildings kind with such a name made the check throw; it is an ordinary problem now. And a system's recompute never throws out into a save: the sheet keeps its values and the server logs why.

New tests fill system files with harmful names at every key and name, nesting and sizes past the caps, and check nothing throws or leaks.

Summary

Test plan

• Tested locally
• Tests pass

Pre-merge checklist

Code quality:

• Code follows project style
• No breaking changes (or clearly documented)
• No console errors or warnings

Version & Release:

• Version bumped? If releasing to users, update:
  • frontend/package.json version
  • docker-compose.yml APP_VERSION
  • CHANGELOG.md with release notes
• GitHub Actions will auto-tag Docker images with the new version

Before merging to main:

• All tests passing
• PR reviewed and approved
• Branch is up to date with main
• No merge conflicts

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