[TheGreatAxios]

Closes CL-11912. Stacked on #156.

What changed

• A name box shows the selection as B2, B2:B5, 3:5 or B:D.
• A read-only box shows the active cell's full contents.
• The status bar shows Average, Count and Sum for the selection.

Verified

• bun run check and bun run build pass on this PR on its own, including the agent references step and the CSV-table a11y sweep.

[linear-code]

CL-11912

[corbits-triage]

Triage: Pass

No changes needed from you. A maintainer will review this pull request.

Stacked on #156: only this pull request's own changes were checked.

Notes:

• added code matches a code pattern: src/lib/csv-grid.ts:213 Invented fallback `?? []`; handle the missing value instead., src/ui/csv-table.tsx:694 Nested ternary; use if statements or a lookup., src/ui/csv-table.tsx:698 Nested ternary; use if statements or a lookup., src/ui/csv-table.tsx:700 Nested ternary; use if statements or a lookup., src/ui/csv-table.tsx:706 Nested ternary; use if statements or a lookup.

Triage will approve, label.