feat(bedit): add Edit Block Definition dialog and new block authoring

Open the Edit Block Definition dialog when BEDIT is invoked without an active selection, listing candidate block definitions alphabetically while filtering layout records, anonymous blocks, and external references. Allow typing a new valid block name to start an empty block editor session that only commits to the drawing block table upon explicit save.

src/modules/draw/modify/block_edit.rs

@@ -70,6 +70,10 @@ pub struct BlockEditSession {
     /// Transient UCS being manipulated in this block tab. On commit its full
     /// frame is baked into the definition and this returns to `None`.
     pub editor_ucs: Option<codec::tables::Ucs>,
+    /// Snapshot of the document prior to creating an empty new block for editing.
+    /// If the session is closed without saving, this snapshot is restored so
+    /// no block record remains in the block table.
+    pub unpersisted_pre_document: Option<codec::CadDocument>,
 }
 
 // ── BEDIT pick command ─────────────────────────────────────────────────────