feat(statusbar): transparency-display toggle (TPY pill)

Add a transparency_enable uniform read by the wire shader: when the new
TPY pill is off, every line is forced opaque; when on, baked entity
transparency shows through. Toggling is a uniform write, no retessellate
(same mechanism as the lineweight-display toggle). Pill is toggleable
from the status-bar customization menu.

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

src/ui/statusbar_config.rs

@@ -23,6 +23,7 @@ pub enum StatusPill {
     Space,
     Scale,
     Units,
+    Transparency,
     Isolate,
     Vp,
     CleanScreen,
@@ -44,6 +45,7 @@ impl StatusPill {
         StatusPill::Space,
         StatusPill::Scale,
         StatusPill::Units,
+        StatusPill::Transparency,
         StatusPill::Isolate,
         StatusPill::Vp,
         StatusPill::CleanScreen,
@@ -64,6 +66,7 @@ impl StatusPill {
             StatusPill::Space => "space",
             StatusPill::Scale => "scale",
             StatusPill::Units => "units",
+            StatusPill::Transparency => "transparency",
             StatusPill::Isolate => "isolate",
             StatusPill::Vp => "vp",
             StatusPill::CleanScreen => "cleanscreen",
@@ -85,6 +88,7 @@ impl StatusPill {
             StatusPill::Space => "Model/Paper Space",
             StatusPill::Scale => "Annotation Scale",
             StatusPill::Units => "Drawing Units",
+            StatusPill::Transparency => "Show Transparency",
             StatusPill::Isolate => "Isolate Objects",
             StatusPill::Vp => "Viewport Count",
             StatusPill::CleanScreen => "Clean Screen",