Merge upstream/main into belt/rebase/may-18

Reconciles the belt-printer branch with upstream PRs through #13723. Six
files had conflicts; three additional files needed manual follow-up fixes
where the auto-merge produced code that referenced upstream-renamed fields
or changed function signatures.

Notable reconciliations:
- TreeSupport.cpp: kept belt-floor early-exit branches around HEAD's
  drop-down logic, folded upstream's `(distance_to_top > 0 ? 1 : 0)`
  formula into the non-belt-floor path (upstream PR #11812). Dropped dead
  `roof_enabled`/`force_tip_to_roof` locals.
- TreeSupport3D.cpp: combined upstream's safety-offset + remove_small
  changes with HEAD's belt-floor clip in the per-slice trim loop. Dropped
  HEAD's `else` block (superseded by upstream's rewritten bottom-contact
  propagation) and re-added the belt-floor clip into the new propagation
  loop. Gated the propagation on belt printers to prevent OOM when
  belt-floor clipping produces empty initial slices.
- TriangleSelector.{cpp,hpp}: merged both new `select_patch` parameters
  (HEAD's `up_direction` and upstream's `select_partially`); body uses
  `dot(up_direction)` for the overhang angle check and forwards
  `select_partially` to `select_triangle`.
- SupportMaterial.cpp: `slicing_params.soluble_interface` →
  `zero_gap_interface_bottom` in HEAD's `detect_belt_floor_bottom_contacts`,
  matching upstream's same-purpose rename at line 2495.
- Custom.json, GCodeWriter.cpp: simple additive merges (kept entries /
  includes from both sides).

Verified by building OrcaSlicer (RelWithDebInfo) after a full deps
rebuild (Eigen v5.0.1, libigl v2.6.0 are now managed deps) and slicing
a scaled Benchy on the NORMALIZER belt-printer profile without OOM.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/slic3r/GUI/GCodeViewer.hpp

@@ -33,7 +33,8 @@ class OpenGLManager;
 static const float GCODE_VIEWER_SLIDER_SCALE = 0.6f;
 static const float SLIDER_DEFAULT_RIGHT_MARGIN  = 10.0f;
 static const float SLIDER_DEFAULT_BOTTOM_MARGIN = 10.0f;
-static const float SLIDER_RIGHT_MARGIN = 124.0f;
+// ORCA: match right margin to the vertical slider window width to prevent overlap.
+static inline const float SLIDER_RIGHT_MARGIN = IMSlider::vertical_slider_window_width();
 static const float SLIDER_BOTTOM_MARGIN = 64.0f;
 class GCodeViewer
 {