feat(pline): create polyline as a live entity so it snaps while drawing

The polyline was only committed to the document on finish, so its partial
geometry produced no snap points — you could not snap to vertices already
placed. Publish it as a real LwPolyline at the second vertex and replace
its geometry in place as later vertices land, so it lives in the document
(and the snapper) throughout the draw.

New CmdResult::CommitLiveEntity / UpdateLiveEntity{finish} plus a
set_live_handle() hook let a command own an entity mid-run. The create
pushes a single undo snapshot; in-place updates push none, so the whole
object reverts as one unit. Updates preserve the entity's handle and
layer (a fresh built entity carries a NULL handle that would desync it
from the document map key and drop it from rendering). (#119)

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

src/command/mod.rs

@@ -225,6 +225,21 @@ pub enum CmdResult {
     /// The host should look up the attdefs for `block_name` from the document
     /// and call `attreq_set_attdefs()` on the command, then loop on text input.
     AttreqNeeded { block_name: String },
+    /// Add a command-owned "live" entity to the document mid-command and hand
+    /// its assigned handle back to the active command via `set_live_handle()`.
+    /// One undo snapshot is pushed here, so the whole in-progress object reverts
+    /// as a single unit. The command stays active. Used by PLINE so the partial
+    /// polyline is a real, snappable entity while later vertices are placed.
+    CommitLiveEntity(EntityType),
+    /// Replace the geometry of the live entity `handle` in place — preserving
+    /// its layer — without pushing a new undo snapshot. When `finish` is true
+    /// the command also exits (the entity is already committed, so no separate
+    /// commit is needed).
+    UpdateLiveEntity {
+        handle: Handle,
+        entity: EntityType,
+        finish: bool,
+    },
 }
 
 /// What kind of value the active command is currently asking for. Drives
@@ -417,6 +432,11 @@ pub trait CadCommand: Send {
         CmdResult::Cancel
     }
 
+    /// Host callback after `CmdResult::CommitLiveEntity`: records the handle the
+    /// new live entity was assigned so later `UpdateLiveEntity` results can
+    /// target it.
+    fn set_live_handle(&mut self, _handle: Handle) {}
+
     /// Point-click pick of domain objects (wire hit-test often misses small markers).
     fn needs_structure_point_pick(&self) -> bool {
         false