ux(grid): cell text cursor, column-type tooltip, Feature Matrix copy #761

Summary

0.4.16 P2 leftovers after Table Browser in-place editing (#759 / #760).

Scope

• Cursor `text` on hover for editable cells (when a staging buffer is attached).
• Cell hover tooltip shows the column SQL type (`name · integer`), plus the existing long-value tooltip.
• README Feature Matrix: `Yes (DML Staging & Preview)` → `Live In-Grid Editing + Staged Commit`.
• F5 refresh is already wired in Table Browser (feat(grid): in-place editing in Table Browser #760); no extra work unless it regressed.

Out of scope

• MongoDB JSON editor (optional P2).
• 120 Hz DevTools QA (qa(motion): Fluid shell + connections tree DevTools sign-off @ 120 Hz #739).

Acceptance

• Editable cells use the text cursor; read-only grids stay `basic`.
• Hover tooltip includes the column type when schema metadata is present.
• README matrix wording updated.
• `flutter test` / `flutter analyze` clean.