feat(system-builder): STATS & RULES page, formulas (4b2d2)

[over2take]

The FORMULAS tab (approved mockup builder-stats-rules, 2026-10-06): each formula's name (blank shows its id), @id and "used by", its formula (conditions shown, not edited, until the graph), and its value for the sample character from api.previewValues, asked PREVIEW_DELAY_MS (400) after typing stops, a late answer never overwriting a newer one. A formula's problem shows under it (aria-invalid, red) while the rest keep their values. INSERT lists @stats, @formulas, tables and functions and puts one at the formula cursor (insertToken). + FORMULA, remove.

StatsRulesPage takes the builder's api; tests work values out with the server's own derived.previewDerived.

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

Related issues