Re-edit: list the bodies as of the feature's timeline slot, not the final ones

Found by sweeping the index-space defect class deliberately rather than by
hitting it: that class produced 4 of the 8 defects found by hand yesterday, so
it was worth auditing every combo in the panel that maps a row selection onto
a document index.

Most of it came back clean — the loft sidecar vectors are consistent at all
four read sites, the sheet-body pickers go through the helper everywhere, mate
connectors carry client data. Six did not. A stored target_body indexes the
body list AS IT WAS just before that feature ran during replay, but Transform,
Mirror, Thicken, Rib, Project and DeleteFace all populated their combo from
the live m_doc.bodies. Boolean and Cut already replayed to the right slot.

The failure is concrete: model a body, Thicken it, then Cut something later in
the tree. A Cut replaces one body with two, so every index at or after it
shifts. Reopen the Thicken and the combo lists the post-cut bodies while
selecting the pre-cut index — showing, and on confirm re-targeting, a
different body than the feature actually used. A Boolean that consumes its
tool body shifts them the other way for the same result.

fill_body_choice() does the truncated replay populate_body_choices() already
did, for the single-combo tools. Six call sites, and 60 lines of duplicated
population loops go with them.

Visible change when testing: re-editing an early feature now lists FEWER
bodies, because it lists only those that existed then. That is correct — you
cannot target a body that did not exist yet — and it is what Boolean and Cut
have always done.

The new kernel test pins the invariant the GUI now leans on: a Cut turns one
body into two, and replaying to just before it yields the earlier, shorter
list. If body ordering after a split ever changes, that assumption fails
loudly here instead of silently in a dialog.

NOT click-tested — GUI wiring, compile-verified only. Filed as snaporca-oz7
and added to snaporca-cfi.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/slic3r/GUI/DesignPanel.hpp

@@ -122,6 +122,10 @@ private:
     // >= 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).
     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
+    // recorded against the body list at that point in the timeline, not the final one.
+    void fill_body_choice(ComboBox* c, int as_of_feature, int want);
     void populate_sheet_body_choices(ComboBox* c) const;   // bodies where is_sheet_shape() is true
     // Rows of a sheet-filtered picker are not body indices; go through these two, never
     // GetSelection()/SetSelection() directly.