feat(ugc): LU Toolbox worker in a headless Blender

UgcToolbox runs LU Toolbox itself in an external Blender that stays up
and makes one model after another (dlu_toolbox_worker.py runs
LU-Toolbox-Standalone's steps: import, Process Model, Bake Lighting,
niftools export). JSON lines over its stdin and original stdout; it is
restarted after a crash, a timeout, 100 models or new settings, runs at
worker_nice with toolbox_threads, and its CPU time is charged to the
worker, which pauses it (SIGSTOP) while the CPU budget is overdrawn.
ProcessModelToolbox reads LU Toolbox's .nif back, counts its triangles
and draws the icon from it. Tests: option parsing, the fallback, the
protocol framing, the worker with a stand-in Blender (crash, hang,
failure, give-up), and LU Toolbox itself when DLU_TEST_TOOLBOX_* are set.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dUgcServer/Processing/UgcThrottle.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <cstdint>
+#include <functional>
 #include <string>
 
 /**
@@ -18,6 +19,10 @@ namespace UgcThrottle {
 	// called, so a job stops in moments when the server shuts down.
 	void Checkpoint();
 
+	// The same, for work done by another process for this thread (LU Toolbox's Blender, charged with Charge first):
+	// `pausing` is called with true before the sleep and false after it, to stop that process meanwhile
+	void Checkpoint(const std::function<void(bool)>& pausing);
+
 	// Thrown by Checkpoint while cancelled: the job is abandoned, not failed (its row stays waiting)
 	struct Cancelled {};
 	// Stop (true) or allow (false) every job's work: set when the server stops, cleared when it starts