Feat: JOIN command — merge collinear Lines or co-circular Arcs

Selection-gathering mode: select objects, press Enter to apply.
- Lines: checks collinearity + no-gap, merges into single line span
- Arcs: checks same center/radius + contiguous angles, merges to Arc or Circle
- Uses CmdResult::JoinEntities; geometry computed in cmd_result from document

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/command/mod.rs

@@ -97,6 +97,8 @@ pub enum CmdResult {
     Measurement(String),
     /// Break `handle` at points `p1` and `p2`; replace with computed fragments.
     BreakEntity { handle: Handle, p1: Vec3, p2: Vec3 },
+    /// Attempt to join the given entities into fewer merged entities.
+    JoinEntities(Vec<Handle>),
 }
 
 // ── Trait ─────────────────────────────────────────────────────────────────