feat(properties): show & edit block attributes in panel

Selecting an INSERT with attributes now adds an editable "Attributes"
section to the Properties panel — each tag is a row whose value can be
changed inline (Enter commits, undo works, the attribute text repaints).
Previously attribute values could only be reached through ATTEDIT, which
didn't let them be changed (#192).

Attribute tags are runtime strings, so they can't ride the geometry
edit path whose field key is `&'static str`; a dedicated PropValue::
AttrText variant plus PropAttrInput/PropAttrCommit messages carry the
tag. Values are stored verbatim (no expression evaluation, since
attribute text is free-form).

Also guard the panel's in-progress edit buffer: it now only carries
across a rebuild when the selection is unchanged (a commit-triggered
refresh), so a typed-but-uncommitted value can't display or commit onto
a different entity — e.g. two title blocks sharing a REV1 tag. This
closes a latent stale-buffer leak that affected geometry fields too.

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

src/app/update/mod.rs

@@ -2001,6 +2001,16 @@ impl OpenCADStudio {
 
             Message::PropGeomCommit(field) => self.on_prop_geom_commit(field),
 
+            Message::PropAttrInput { tag, value } => {
+                self.tabs[self.active_tab]
+                    .properties
+                    .edit_buf
+                    .insert(crate::ui::properties::attr_edit_key(&tag), value);
+                Task::none()
+            }
+
+            Message::PropAttrCommit(tag) => self.on_prop_attr_commit(tag),
+
             Message::PropColorPickerToggle => {
                 let i = self.active_tab;
                 self.tabs[i].properties.color_picker_open =