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/scene/model/object.rs

@@ -31,6 +31,10 @@ pub enum PropValue {
     BoolToggle { field: &'static str, value: bool },
     /// Hatch pattern name — rendered as a combo_box from the catalog.
     HatchPatternChoice(String),
+    /// Block attribute value keyed by its (dynamic, runtime) tag — rendered as
+    /// an editable text_input. Unlike the other rows the routing key is the
+    /// tag carried here, not the row's `&'static str` field.
+    AttrText { tag: String, value: String },
 }
 
 /// A single property row in the Properties panel.