Tune EU7 PACK streaming with v10 CHNK sub-chunks and worker read cache

Chunk PACK sections at bake time and decode them incrementally at runtime
to cut apply hitches during fast camera flight; cache parsed section headers
per worker thread to avoid reopening and reparsing on every sub-chunk read.

scene/eu7/eu7_pack_bench.h

@@ -20,6 +20,7 @@ namespace scene::eu7 {
 struct Eu7PackBench {
     // --- worker (async read PACK) ---
     double worker_read_pack_ms { 0.0 };
+    std::uint64_t worker_chunks_decoded { 0 };
     std::uint64_t worker_sections_done { 0 };
     std::uint64_t sections_finalized { 0 };
     std::uint64_t worker_models_decoded { 0 };