Match Fix geometric constraint behavior

1) FXCONSTRAINT (and the new GCFIX name) prompts "Select point or [Object]
   <Object>:" — a pick fixes that constraint point; Enter or O asks for one
   line, polyline segment, circle, arc, ellipse or spline; misses and
   unsupported objects report the reference messages and re-prompt.
2) Point, block, text, MText, attribute and table insertion points are
   addressable constraint points.
3) Re-applying Fixed to the same point or object reports that the constraint
   already exists instead of adding a duplicate.
4) A persistent Fixed holds its pre-edit geometry through MOVE, ROTATE,
   SCALE, STRETCH, grip and Properties edits: fixed points stay, free points
   of a partly fixed entity follow the edit, and size retention skips fixed
   entities.
5) Object mode on a polyline fixes the picked segment (natively persisted)
   instead of the whole polyline; arc segments pin endpoints, center and
   radius.
6) Ribbon label "Fix" with a padlock icon.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

assets/icons/constrain/fixed.svg

@@ -1,7 +1,5 @@
 <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="64" height="64">
-  <line x1="4" y1="8" x2="20" y2="8" stroke="#6DB7ED" stroke-width="2.25" stroke-linecap="round"/>
-  <line x1="6" y1="8" x2="3" y2="13" stroke="#B4B6B9" stroke-width="1.6" stroke-linecap="round"/>
-  <line x1="11" y1="8" x2="8" y2="13" stroke="#B4B6B9" stroke-width="1.6" stroke-linecap="round"/>
-  <line x1="16" y1="8" x2="13" y2="13" stroke="#B4B6B9" stroke-width="1.6" stroke-linecap="round"/>
-  <line x1="21" y1="8" x2="18" y2="13" stroke="#B4B6B9" stroke-width="1.6" stroke-linecap="round"/>
+  <path d="M8 10.5V8a4 4 0 0 1 8 0v2.5" fill="none" stroke="#B4B6B9" stroke-width="2" stroke-linecap="round"/>
+  <rect x="5.5" y="10.5" width="13" height="9.5" rx="1.5" fill="none" stroke="#6DB7ED" stroke-width="2.25"/>
+  <circle cx="12" cy="15.25" r="1.25" fill="#6DB7ED"/>
 </svg>