Audit remaining Tabular Phase 2 improvements

@paullizer is already working on this.

Since Jun 26, 2026.

## Summary
Review the current tabular pipeline and decide what Phase 2 still means: larger files, better workbook sheets, deterministic charting, generated exports, evidence joining, or UI polish.

## User Value
Tracking this as a GitHub issue moves the backlog item out of a private markdown file and gives the team a clear place to prioritize, design, implement, and validate the work.

## Proposed Behavior
Start with a short audit issue rather than an implementation issue. The repo already has substantial tabular functionality, so avoid duplicating completed work.

## Acceptance Criteria
- [ ] The requested behavior is designed or implemented for the relevant SimpleChat surfaces.
- [ ] Permission, configuration, and existing-feature interactions are accounted for.
- [ ] Tests, documentation, or validation notes are added as appropriate.

## Notes
- Source backlog entry: Tabular Phase 2
- Source file: priv-simplechat-2/grouped_feature_fix_backlog.md
- Backlog status: Research / Extension
- Current anchors: multiple tabular docs under `docs/explanation/features/`, including background exports, chat-scoped exports, related document evidence, and inline chart handoff.
- Proposed roadmap priority: P2
- Proposed roadmap size: M
- Related: #802 tracks moving tabular orchestration out of route_backend_chats.py. This issue is an audit/design pass for remaining Tabular Phase 2 improvements.