Add headless parallel eu7v2 scenario bake with streaming and PLCE placements

Enable --eu7v2-bake from the main binary: parallel module pool, bounded-RAM
spool flush, streaming terrain triangles, flat include/model parsing, and
eu7v2 emit/load with optional verify. Large placement .scm files emit lean
PLCE records and bake referenced .inc modules separately for reuse.

- CLI: --eu7v2-bake, --eu7v2-verify, --eu7v2-mem-limit-gb, --eu7v2-threads,
  --eu7v2-max-parse; wire max_threads through to the bake parser
- eu7v2 v2 records: PLCE placements, runtime emitter/loader, batch verify
- Parallel bake pool with session cache; drop heavy-serial parse gate in spool
  mode; parse concurrency matches thread count
- Streaming terrain: batched parallel parse+bake, scan/bake pipeline, shape
  spool with persistent buffered I/O and flush-before-read
- Parallel flat-file streaming for models/includes; pack/model spool for
  low-memory incremental flush
- Optional 50 GB private-bytes guard during headless bake

Braniewo_szeroki: 160 modules, verify PASS, ~34s bake (nmt100 ~17s vs ~190s
serial baseline).

Co-authored-by: Cursor <cursoragent@cursor.com>

scene/eu7/eu7_load_stats.h

@@ -52,6 +52,8 @@ struct Eu7LoadStats {
     std::uint64_t sounds { 0 };
     std::uint64_t events { 0 };
     std::uint64_t trainsets { 0 };
+    std::uint64_t module_v2_loaded { 0 };
+    std::uint64_t module_v2_baked { 0 };
     std::uint64_t pack_skipped_includes { 0 };
     std::uint64_t pack_sections_loaded { 0 };
     std::uint64_t pack_models { 0 };