Improve GridForge spreadsheet editing

[linuxfold]

Summary

This PR turns GridForge into a much more functional spreadsheet app by improving the grid renderer, formula editing workflow, workbook mutation model, XLSX preservation, launch behavior, and packaging flow.

Major changes

• Reworked the grid renderer around separate body/header/corner views to reduce scroll-copy artifacts and visual tearing.
• Added a richer formula reference model with CellReference and range-aware formula parsing/evaluation support.
• Added Excel-like formula entry behavior:
  • type =, click cells to insert references
  • drag ranges such as B7:B12
  • color-code formula references and referenced ranges
  • edit formulas directly in the formula bar
  • auto-close a single missing parenthesis when it makes the formula valid
• Improved keyboard behavior:
  • grid gets focus on startup
  • arrow keys work after edits
  • arrow keys commit inline cell edits and move selection
  • removed body-cell hover tint that caused stale gray artifacts
• Moved visible workbook mutations toward command-based operations for cleaner undo/redo and dirty tracking.
• Expanded command support for edits, formatting, sheet operations, row/column operations, paste, and resize workflows.
• Added XLSX package preservation scaffolding so unsupported workbook content can be retained more safely.
• Improved app/document scaffolding with GridForgeDocument.
• Improved build packaging:
  • ad-hoc signing fallback when Developer ID signing is unavailable
  • DMG generation still succeeds for local testing
  • .xlsx UTI declarations
  • safer notarization password handling
• Added/updated model coverage for the new workbook/reference behavior.

Validation

• swift build passes
• ./build.sh build passes
• ./build.sh dmg passes
  Note: swift test could not run in this local environment because the toolchain cannot load XCTest (no such module 'XCTest').