ifcviewer: extract ChunkPlanner + InstanceCompose; add Tier-1 test trio

The chunk planner (Morton sort + greedy pack) and instance composition
(federation × placement matrix chain + world-AABB derive) were inline
helpers in ViewportWindow.cpp. Pulled both out as free-function modules
so the math + lookup logic can be exercised without a Qt window or a
wgpu device. ViewportWindow now delegates; InstanceLookup is a using-
alias to InstanceCompose::InstanceLookup.

Also added an addSubBufferForTesting / clearSubPoolsForTesting seam to
BufferPool so the sub-allocator invariants can be pinned with fake
WGPUBuffer handles. The fakes are never dereferenced; the guard drops
the sub-pools before destructor would call wgpuBufferRelease.

Three new test binaries under src/ifcviewer/tests/, 33 cases / 173
assertions: BufferPool first-fit + alignment + coalescing + multi-
sub-pool isolation; ChunkPlanner Morton split / interleave / stable
sort / greedy-pack monotonicity and single-mesh-oversize; InstanceCompose
identity / translation / order-of-multiplication / large-placement
cancellation against federation false origin / column-major writeback /
findInstance lookup paths.

src/ifcviewer/BufferPool.cpp

@@ -223,3 +223,17 @@ uint64_t BufferPool::largest_free_run_bytes() const {
     }
     return m;
 }
+
+void BufferPool::addSubBufferForTesting(WGPUBuffer fake_buffer, uint64_t capacity) {
+    SubPool sp;
+    sp.buffer   = fake_buffer;
+    sp.capacity = capacity;
+    sp.used     = 0;
+    sp.free_ranges.push_back({0, capacity});
+    sub_pools_.push_back(std::move(sp));
+}
+
+void BufferPool::clearSubPoolsForTesting() {
+    // Skip wgpuBufferRelease — handles are fakes that would crash on deref.
+    sub_pools_.clear();
+}