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/mod.rs

@@ -1385,6 +1385,11 @@ pub enum Message {
     },
     /// User committed a geometry/common field edit (Enter pressed).
     PropGeomCommit(&'static str),
+    /// User is typing in a block-attribute value field (live buffer update),
+    /// keyed by the attribute tag.
+    PropAttrInput { tag: String, value: String },
+    /// User committed a block-attribute value edit (Enter pressed).
+    PropAttrCommit(String),
     /// Toggle the inline color picker dropdown open/closed.
     PropColorPickerToggle,
     /// Toggle the full ACI colour palette expansion.