tui: unify layout through boxed Grid subgrids

[Michael Greene (mgreenegit)]

Review boundary

Second slice: the lasting shared layout engine, with application callers unchanged. Review/merge order: #950 -> #961 -> #962.

Review only this slice: 494 additions / 198 deletions across two files (692 changed lines, down from 742). The comparison uses a content-identical linear review snapshot; this PR itself retains one cumulative commit against main, including unmerged #950.

Changes

• Replace the separate table engine with thin declarations over Grid.
• Boxed column subgrids share tracks while preserving row IDs, full-width backgrounds/hitboxes, borders, padding, and focus.
• Add intrinsic tracks, implicit sizing, gaps, item alignment, and layout-independent arrow traversal. Preserve existing Grid defaults and descendant input priority.
• Keep the final engine and generic tests here; tui: migrate remaining layouts to Grid #962 migrates callers and removes wrappers without replacing either.
• Extend Introduce native CSS-style Grid for the editor layout #950's private item-placement helper; compute only the horizontal insets shared columns consume. The TUI-only pass reduces this slice by 19 changed lines without changing tests.

This is a terminal Grid subset: subgrids occupy a complete parent row; nested column subgrids and arbitrary spans are unsupported. Ordinary nested grids work. Row borders/padding contribute to sizing and clip cells to row interiors.

This pull request was drafted by a large language model (LLM).

Validation

• Latest local validation: nine Grid tests and two application layout tests pass, along with formatting, pre-commit workspace Clippy, and scoped structural checks.
• Final-head Windows/Ubuntu CI and CodeQL pass at d333592.
• One combined geometry fixture checks minimum/implicit columns, gaps, empty/ragged rows, floats, cell and row borders, clipping, unequal heights, and collapse/re-expansion. Separate tests retain mixed flat/shared rows, alignment, navigation, input priority, and menu hitboxes.

No dependencies, Flexbox, stylesheet parser, console, provider, or AI changes.