Feat: LENGTHEN command (LEN) — extend or trim Line/Arc

Options via text input after entity pick:
  DE <delta>   — extend by delta (negative trims)
  TO <total>   — set absolute total length / arc length
  P <percent>  — scale by percentage
  <number>     — plain number treated as DE delta

Closest end to pick point is modified. Uses CmdResult::LengthenEntity.

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

src/command/mod.rs

@@ -103,6 +103,12 @@ pub enum CmdResult {
     DivideEntity { handle: Handle, n: usize },
     /// Place Point entities at `segment_length` intervals along the entity.
     MeasureEntity { handle: Handle, segment_length: f64 },
+    /// Extend/trim a Line or Arc by the given mode; end command.
+    LengthenEntity {
+        handle: Handle,
+        pick_pt: Vec3,
+        mode: crate::modules::home::modify::lengthen::LenMode,
+    },
 }
 
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