perf(statusbar): cache layout/scale derived data behind epochs (major)

Status bar rescanned the whole object map twice per frame (layout_names +
scale_picker_list), deep-cloned an already-cached Arc, and built two
undo/redo label vecs consumed ~never. Cache layout names (epoch key) and
scale picker (epoch + unit-factor + imperial-family + current-scale key)
as Arc behind choke-point invalidation (layout/scale impl methods,
undo/redo swaps, file-open, doc-reset); borrow across the widget boundary
(owned-Arc params; E0515 forbids &'a slices over call-site temporaries);
build undo/redo labels only when their dropdown is open.

Tests:
- cached_layout_names_returns_same_arc_until_bump: hit-path Arc identity
- cached_layout_names_invalidated_by_add_layout: bump yields new Arc + names
- cached_scale_picker_returns_same_arc_until_bump: hit-path Arc identity
- cached_scale_picker_invalidated_by_add_scale: bump yields new Arc + scale
- cached_scale_picker_invalidated_by_current_scale_change: key completeness

Bench (cargo bench ui_statusbar_derived_data, 3k entities): cached hit below
timer granularity vs ~1us uncached scans, PASS (50us threshold).

src/ui/ribbon/mod.rs

@@ -25,6 +25,7 @@ mod draw_panel;
 mod modify_panel;
 mod color_dropdown;
 use widgets::{StyleContext, *};
+pub(crate) use widgets::{REDO_HISTORY_ID, UNDO_HISTORY_ID};
 mod collapse;
 use collapse::{CollapsePanels, Panel};
 pub use collapse::CollapseMode;

src/ui/ribbon/widgets.rs

@@ -301,8 +301,8 @@ pub(super) const TOP_HIST_GAP: f32 = 4.0;
 
 // ── Dropdown / combo ID constants ─────────────────────────────────────────
 
-pub(super) const UNDO_HISTORY_ID: &str = "UNDO_HISTORY";
-pub(super) const REDO_HISTORY_ID: &str = "REDO_HISTORY";
+pub(crate) const UNDO_HISTORY_ID: &str = "UNDO_HISTORY";
+pub(crate) const REDO_HISTORY_ID: &str = "REDO_HISTORY";
 pub(super) const LAYER_COMBO_ID: &str = "LAYER_COMBO";
 /// Dropdown id for the tab-bar panel-density selector.
 pub(super) const COLLAPSE_MODE_ID: &str = "COLLAPSE_MODE";