Design: a body tool acts on the body you picked, not on the first one

Mirror of snaporca e5e223a794 (DesignPanel.cpp applied as a patch; parity 30, hpp
byte-identical).

Every body combo opened on index 0, so picking a body and pressing Mirror acted on a
different solid while the card showed that other body as the target. Nine sites now read
the viewport selection; Boolean takes the picked body as target and a different one as
tool, since defaulting both to the same body is a no-op.

Verified functionally on the rig: picked the 20x20 body, mirrored, and the new body
measures 20x20 — not the 80x50 one it would have used before.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LyRwbuq6fjn3VV9U9UvhBM

src/slic3r/GUI/DesignPanel.hpp

@@ -131,6 +131,11 @@ private:
     // Fill m_bool_target / m_bool_tool / m_cut_target. as_of_feature < 0 = current bodies (add);
     // >= 0 = the bodies as they existed just before that feature index (Boolean re-edit, so a
     // consumed tool body still appears and its saved selection round-trips).
+    // Which body a tool should act on when it opens: the one picked in the VIEWPORT, else
+    // the first. Selection comes first and the tool consumes it — every body combo used to
+    // default to index 0, so picking body 3 and opening Mirror silently mirrored body 1.
+    // Clamped to the list, so it is safe to hand straight to SetSelection. snaporca-e1p.
+    int  selected_body_default() const;
     void populate_body_choices(int as_of_feature = -1);
     // Fill `c` with the bodies as they existed just before `as_of_feature` and select
     // `want`. Re-editing any feature that stores a body index needs this: the index was