refactor!: unify records and pivot mode cell handling

Refactor records mode to use synthetic CellKeys (_Index, _Dim) for all columns,
allowing uniform handling of display values and edits across both pivot and
records modes.

- Introduce `synthetic_record_info` to extract metadata from synthetic keys.
- Update `GridLayout::cell_key` to return synthetic keys in records mode.
- Add `GridLayout::resolve_display` to handle value resolution for synthetic
  keys.
- Replace `records_col` and `records_value` in `CmdContext` with a unified
  `display_value`.
- Update `EditOrDrill` and `AddRecordRow` to use synthetic key detection.
- Refactor `CommitCellEdit` to use a shared `commit_cell_value` helper.

BREAKING CHANGE: CmdContext fields `records_col` and `records_value` are replaced by
`display_value` .
Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-31B-it-GGUF:UD-Q5_K_XL)

src/ui/effect.rs

@@ -97,15 +97,7 @@ pub struct EnterEditAtCursor;
 impl Effect for EnterEditAtCursor {
     fn apply(&self, app: &mut App) {
         let ctx = app.cmd_context(crossterm::event::KeyCode::Null, crossterm::event::KeyModifiers::NONE);
-        let value = if let Some(v) = &ctx.records_value {
-            v.clone()
-        } else {
-            ctx.cell_key
-                .as_ref()
-                .and_then(|k| ctx.model.get_cell(k).cloned())
-                .map(|v| v.to_string())
-                .unwrap_or_default()
-        };
+        let value = ctx.display_value.clone();
         drop(ctx);
         app.buffers.insert("edit".to_string(), value);
         app.mode = AppMode::Editing {