Add UCS FACE and UCS OBJECT

Draw a profile, extrude it, then sketch on a face of the result: the
operations were all there, but nothing turned a solid's face into the
active drawing plane, so the second half of that loop was unreachable.

UCS FACE takes a surface pick, finds the planar face under it and
builds the plane from the face normal. X comes from the Arbitrary Axis
Algorithm, so the same face always yields the same X axis and a sketch
drawn on it does not spin when the UCS is rebuilt.

UCS OBJECT adopts a planar entity's own stored axes instead of
re-deriving X, so a sketch started from a circle lines up with how
that circle is stored. It reuses the profile extraction EXTRUDE
already runs, which means anything extrudable can also be sketched on.

Bare UCS FACE starts an interactive pick that resolves through the
same inline handler, so the clicked and typed paths cannot diverge.

src/command.rs

@@ -347,6 +347,62 @@ impl CadCommand for ValuePromptCommand {
     }
 }
 
+/// Interactive front-end for `UCS FACE` — Fusion's "create sketch on a face".
+///
+/// Picks one face of a solid and hands the resulting plane to the inline
+/// `UCS FACE <handle> <x,y,z>` handler via [`CmdResult::Dispatch`], so the
+/// plane construction lives in exactly one place whether the user clicked a
+/// face or typed the arguments. `entity_pick_uses_surface_point` is what
+/// makes the click land *on the solid's surface* rather than on the current
+/// drawing plane, which is the whole point: the pick coordinate is the face.
+pub struct UcsFaceCommand;
+
+impl CadCommand for UcsFaceCommand {
+    fn name(&self) -> &'static str {
+        "UCS"
+    }
+
+    fn prompt(&self) -> String {
+        crate::t!("UCS FACE  Select a face to draw on:").into_owned()
+    }
+
+    fn needs_entity_pick(&self) -> bool {
+        true
+    }
+
+    fn entity_pick_uses_surface_point(&self) -> bool {
+        true
+    }
+
+    fn entity_pick_highlights_hover(&self) -> bool {
+        true
+    }
+
+    fn on_entity_pick(&mut self, handle: Handle, pt: DVec3) -> CmdResult {
+        if handle.is_null() {
+            return CmdResult::NeedPoint;
+        }
+        // Hex handle and comma-separated coordinates are what the inline
+        // parser reads back. Full `{}` precision, not a rounded format: the
+        // point has to stay on the face for the planar-face lookup.
+        CmdResult::Dispatch(format!(
+            "UCS FACE {:X} {},{},{}",
+            handle.value(),
+            pt.x,
+            pt.y,
+            pt.z
+        ))
+    }
+
+    fn on_point(&mut self, _pt: DVec3) -> CmdResult {
+        CmdResult::NeedPoint
+    }
+
+    fn on_enter(&mut self) -> CmdResult {
+        CmdResult::Cancel
+    }
+}
+
 /// Interactive front-end for RENAME. Prompts for the object type (as clickable
 /// buttons), then the current name, then the new name, and delegates to the
 /// inline `RENAME <type> <old> <new>` handler via [`CmdResult::Dispatch`] — the