feat(block-editor): edit definition properties in the Properties panel

Add the no-selection Block section with annotation, orientation, scaling, exploding, units, description and hyperlink controls. Integrate definition edits with editor history, save checkpoints and discard, and keep staged text tied to its definition.

Persist annotation and hyperlink metadata on the block record, migrate legacy marker values on edit, and honor orientation and hyperlink descriptions during block creation. Cover native DWG/DXF roundtrips and replacement after reopening.

src/ui/properties.rs

@@ -595,6 +595,8 @@ pub struct PropertiesPanel {
     /// commit onto a different entity (e.g. two title blocks sharing a `REV1`
     /// tag).
     pub source_handles: Vec<Handle>,
+    /// Definition shown by the no-selection Block section, separate from entity targets.
+    pub block_definition: Option<Handle>,
     /// Whether the quick color picker dropdown is open.
     pub color_picker_open: bool,
     /// Whether the MTEXT background-colour picker dropdown is open. Separate
@@ -657,6 +659,7 @@ impl Default for PropertiesPanel {
             hatch_pattern_focus: 0,
             edit_buf: HashMap::default(),
             source_handles: vec![],
+            block_definition: None,
             color_picker_open: false,
             bg_color_picker_open: false,
             open_color_field: None,