feat: navigate table rows vertically

[Happypig375]

Summary

• navigate vertically between rows of table-like structures while preserving recursive editor paths
• select the nearest caret in ragged rows using display geometry, with deterministic model-path fallback where a backend cannot index table row paths
• skip empty rows, clamp ragged columns, and preserve enclosing fraction/script/radical/inner/nested-table paths
• harden root atom lookup for invalid negative indices without changing left/right behavior

Testing

• full KeyPressTests: 326 passed
• full Core test project (implementation check): 1,608 passed
• Core test project build passed with 0 warnings/errors
• scoped formatting verification and git diff --check passed

Stack

This PR is based on issue54/NavigationByUpDownArrows / #124 because table navigation extends its up/down editor model.

Closes #286