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.h

@@ -111,6 +111,20 @@ public:
     // could rescue them, or whether eviction is the only path.
     bool     can_grow() const { return !growth_disabled_ && per_sub_buffer_capacity_ > 0; }
 
+    // Test-only seam. Production code populates sub-pools lazily through
+    // alloc() → addSubBuffer() → wgpuDeviceCreateBuffer; that path needs a
+    // real WGPUDevice and is impractical to exercise from a unit test.
+    // tests/test_buffer_pool.cpp uses this method to preseed a sub-pool
+    // with a known capacity and a fake (non-null) WGPUBuffer handle the
+    // allocator only treats as opaque — the free-list bookkeeping never
+    // dereferences it. Not for production use.
+    void addSubBufferForTesting(WGPUBuffer fake_buffer, uint64_t capacity);
+    // Drop fake-handle sub-pools without calling wgpuBufferRelease on
+    // them. Tests must call this before the pool destructs (or rely on
+    // the FakePoolGuard fixture in test_buffer_pool.cpp), otherwise
+    // ~BufferPool → destroy() would dereference the fake handles.
+    void clearSubPoolsForTesting();
+
 private:
     struct FreeRange { uint64_t offset; uint64_t size; };
     struct SubPool {