Add table slicers (insert.slicer / table.insertSlicer)

Implements the two slicer commands from the parity catalog as first-class
floating objects that filter a table column.

- model: Slicer struct on Sheet (#[serde(default)]), included in
  Workbook::next_object_id; slicer anchors use AnchorMode::MoveOnly.
- engine: cmd/slicer.rs with insert.slicer, table.insertSlicer, slicer.toggle
  (single + multi), slicer.selectAll, slicer.clear, slicer.delete. Selection is
  written as a FilterCriterion::Values into the sheet AutoFilter and applied via
  the existing apply_filter. object.move / object.delete handle "slicer".
- ui-egui: slicers rendered, hit-tested, selected and dragged like charts; value
  tiles toggle on click (Ctrl/Cmd = multi), funnel button clears. Ribbon buttons
  under Insert > Filters and Table Design > Tools.
- tests: slicer_filters_table; the every_command_survives_empty_params fuzz
  covers the new commands.

Native-only for now; .xlsx slicer-cache round-trip is a follow-up.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/engine/src/cmd/insert.rs

@@ -869,6 +869,7 @@ fn delete_object(s: &mut Session, p: &Json) -> Result<Json> {
         match kind.as_str() {
             "chart" => sh.charts.retain(|c| c.id != id),
             "image" => sh.images.retain(|c| c.id != id),
+            "slicer" => sh.slicers.retain(|c| c.id != id),
             _ => sh.shapes.retain(|c| c.id != id),
         }
         Ok(Json::Null)
@@ -884,6 +885,7 @@ fn move_object(s: &mut Session, p: &Json) -> Result<Json> {
         let anchor = match kind.as_str() {
             "chart" => sh.charts.iter_mut().find(|c| c.id == id).map(|c| &mut c.anchor),
             "image" => sh.images.iter_mut().find(|c| c.id == id).map(|c| &mut c.anchor),
+            "slicer" => sh.slicers.iter_mut().find(|c| c.id == id).map(|c| &mut c.anchor),
             "shape" => sh.shapes.iter_mut().find(|c| c.id == id).map(|c| &mut c.anchor),
             _ => None,
         };