Add EU7 PACK v12 PROT/INST and tune streaming apply path

Introduce EU7B v8 bake with global PROT chunk and v12 section payloads
(solo MODL + compact inst records, per-chunk solo/inst tables). Reader
parses v12 with chunked decode and v11→flat fallback; fix chunk blob
layout to write solo blocks before inst records.

Streaming: add unload keep-radius hysteresis on deceleration, ring-deficit
urgent drain, higher cold-mesh/apply budgets, and smarter warm-prefix
preload. Move UMES mesh prefetch to the main thread and route PACK apply
through ensure_pack_mesh_in_session_cache to avoid GetModel races on
Global.asCurrentTexturePath; harden MdlMngr texture-path trimming.

Rebake root scenario .eu7 files to get v12; existing v7–v11 packs remain
readable.

scene/eu7/eu7_pack_bench.h

@@ -71,6 +71,9 @@ struct Eu7PackBench {
     std::uint64_t stream_dequeue_camera { 0 };
     std::uint64_t stream_dequeue_wait_near { 0 };
     std::uint64_t stream_jobs_blocked_far { 0 };
+    std::uint64_t stream_jobs_blocked_ring_inner { 0 };
+    std::uint64_t stream_jobs_blocked_ring_outer { 0 };
+    std::uint64_t stream_worker_deferred_distant { 0 };
     std::uint64_t stream_apply_stuck_skip { 0 };
     std::uint64_t stream_sections_unloaded { 0 };
     std::uint64_t stream_unload_instances { 0 };