feat(block): BEDIT block editor as a dedicated space tab (#261)

Selecting a block and running BEDIT (command, ribbon button, or a
double-click on the block) opens a block-editor "space" tab, named after
the block, alongside Model/Layout in the status bar. The tab draws ONLY
the block record's own entities in block-local coordinates; edits are
live on the definition, so every INSERT updates. Save Block / Discard
buttons (side toolbar) finish the session; Discard restores the on-entry
snapshot. Space switching is locked while a block is open, and BEDIT and
REFEDIT are mutually exclusive.

Implementation: a Scene.block_edit_block override at the top of
current_layout_block_handle scopes render/pick/new-entity ownership to the
edited block while current_layout stays "Model" (so the pervasive
Model-vs-paper code keeps model behaviour). The model render/pick paths
that hardcoded model_space_block_handle (wire tile set, hatch render +
pick, block-instance meshes) are made binding-aware so the block isolation
actually takes effect and model content does not leak in. Entering fits
the camera to the block (fit_all already scopes via the binding). Renaming
the block tab renames the block itself — new Scene::rename_block re-keys
the record, updates the Block marker and repoints every INSERT reference.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/document.rs

@@ -1,5 +1,6 @@
 use crate::command::CadCommand;
 use crate::io::linetypes;
+use crate::modules::draw::modify::block_edit::BlockEditSession;
 use crate::modules::draw::modify::refedit::RefEditSession;
 use crate::scene::pick::grip::GripEdit;
 use crate::scene::GripDef;
@@ -205,6 +206,8 @@ pub(super) struct DocumentTab {
     pub(super) paper_bg_color: Option<[f32; 4]>,
     /// Active REFEDIT session, if any.
     pub(super) refedit_session: Option<RefEditSession>,
+    /// Active BEDIT block-editor space session, if any (issue #261).
+    pub(super) block_edit: Option<BlockEditSession>,
     /// Currently active MLeader style name.
     pub(super) active_mleader_style: String,
     /// Last camera_generation value written back to the document.
@@ -437,6 +440,7 @@ impl DocumentTab {
             bg_color: None,
             paper_bg_color: None,
             refedit_session: None,
+            block_edit: None,
             active_mleader_style: "Standard".to_string(),
             last_synced_camera_gen: 0,
             is_start: false,