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  HEAD's `else` block (superseded by upstream's rewritten bottom-contact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/libslic3r/CMakeLists.txt

@@ -500,7 +500,7 @@ if (APPLE)
         )
 endif ()
 
-add_library(libslic3r STATIC ${lisbslic3r_sources} 
+add_library(libslic3r STATIC ${lisbslic3r_sources}
     "${CMAKE_CURRENT_BINARY_DIR}/libslic3r_version.h"
     ${OpenVDBUtils_SOURCES})
 source_group(TREE ${CMAKE_CURRENT_SOURCE_DIR} FILES ${lisbslic3r_sources})
@@ -516,7 +516,7 @@ find_package(CGAL REQUIRED)
 find_package(OpenCV REQUIRED core)
 cmake_policy(POP)
 
-add_library(libslic3r_cgal STATIC 
+add_library(libslic3r_cgal STATIC
     CutSurface.hpp CutSurface.cpp
     IntersectionPoints.hpp IntersectionPoints.cpp
     MeshBoolean.hpp MeshBoolean.cpp
@@ -541,7 +541,7 @@ if (_opts)
     target_compile_options(libslic3r_cgal PRIVATE "${_opts_bad}")
 endif()
 
-target_link_libraries(libslic3r_cgal PRIVATE ${_cgal_tgt} eigen admesh libigl mcut boost_libs)
+target_link_libraries(libslic3r_cgal PRIVATE ${_cgal_tgt} admesh libigl mcut boost_libs)
 
 if (MSVC AND "${CMAKE_SIZEOF_VOID_P}" STREQUAL "4") # 32 bit MSVC workaround
     target_compile_definitions(libslic3r_cgal PRIVATE CGAL_DO_NOT_USE_MPZF)
@@ -592,6 +592,7 @@ set(OCCT_LIBS
 
 target_link_libraries(libslic3r
     PUBLIC
+        Eigen3::Eigen
         admesh
         libigl
         libnest2d
@@ -606,7 +607,6 @@ target_link_libraries(libslic3r
         clipper
         Clipper2
         draco::draco
-        eigen
         glu-libtess
         JPEG::JPEG
         libslic3r_cgal