fix(properties): scope asym-scale opt-out per tab

props_asym_scale was app-global keyed by raw handle values, but fresh documents restart numbering at seed 1, leaking one tab's unchecked Uniform box into another tab reusing the value. The set moves to DocumentTab; all uses tab-scoped. Also removes a dead destructure left in overlay.rs. Test: asym_scale_opt_out_does_not_leak_across_tabs with handle-reuse premise assert (RED first). Full suite: 2025 passed / 0 failed / 25 ignored, zero warnings.

src/app/mod.rs

@@ -419,10 +419,6 @@ pub(super) struct OpenCADStudio {
     discussions: Vec<crate::discussions::DiscussionEntry>,
     /// True while the boot-time Discussions refresh is still in flight.
     discussions_loading: bool,
-    /// Block references whose properties panel shows per-axis Scale X/Y/Z even
-    /// though the three factors are currently equal — the user unchecked the
-    /// "Uniform scale" box for them (#427). Keyed by entity handle.
-    props_asym_scale: std::collections::HashSet<u64>,
     /// Collapsed Properties-panel section titles. This belongs to the app,
     /// rather than an individual document tab, so the same view preference is
     /// used by every currently open drawing/project.
@@ -4110,7 +4106,6 @@ impl OpenCADStudio {
             videos_loading: false,
             discussions: Vec::new(),
             discussions_loading: false,
-            props_asym_scale: std::collections::HashSet::new(),
             collapsed_property_sections: rustc_hash::FxHashSet::default(),
             start_section: StartSection::default(),
             start_action_w: std::sync::Arc::new(std::sync::atomic::AtomicU32::new(0)),