* Add Missing Includes Across src/libslic3r
Every libslic3r source and header now directly includes the headers declaring what it uses, rather than relying on the precompiled header or transitive includes. Generated with clang-tidy misc-include-cleaner, with libslic3r headers spelled libslic3r/... so they resolve outside the library's private include paths. MultiMaterialSegmentation.hpp, Support/SupportParameters.hpp and Format/STEP.hpp are made self-contained by hand.
* Make the libslic3r Headers Compile on Their Own
Each now includes, or forward-declares, what it uses instead of relying on what its includers happened to include first. Left out: I18N.hpp, which errors on purpose when included from GUI code, and VoxelizeCSGMesh.hpp and SLA/bicubic.h, which nothing includes and which no longer compile at all.
* Add the Includes Missing From the Hand-Fixed libslic3r Headers
clang-tidy would not edit these headers while they failed to compile on their own, so the first pass skipped them. With the headers now self-contained, a second pass adds the rest.
* Keep Windows Setup Ahead of the Added libslic3r Includes
Print.cpp and Thread.cpp open with a _WIN32 block that has to come first; without the precompiled header, Print.cpp otherwise reaches windows.h through OCCT with NONLS defined and boost/regex fails. OpenVDBUtils.cpp and SLA/SupportTreeBuilder.cpp had includes inside #ifndef NOMINMAX, which libslic3r defines on Windows, so those were skipped there. .clang-tidy also ignores the MSVC STL and UCRT internals, Boost.Multiprecision's fwd.hpp and CPython's Windows include directory.
* Re-Add libslic3r Includes After the Clipper2 2.0.1 Migration
Rebasing onto main took main's version of the files the Clipper2 migration rewrote, so their added includes are restored here, along with includes for main's new code. Clipper2's individual headers are now ignored by clang-tidy: they only build the Z variant through clipper2_z.hpp, which defines USINGZ first, so including clipper.core.h and the like directly broke ClipperZUtils.cpp.
A build without SLIC3R_PCH had never been tried on macOS. Three files
used what pchheader.hpp happened to include: LocalesUtils.cpp needs
<sstream> and <iomanip>, and the two dialogs need <wx/tooltip.h>. The
GTK port's headers and libstdc++ pull these in transitively, the Cocoa
port's headers and libc++ do not.
