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.cpp

@@ -80,7 +80,9 @@ log_load_stats() {
     WriteLog(
         "  read/cache: " + format_seconds( s.read_ms ) +
         " reads=" + std::to_string( s.module_read ) +
-        " cache_hit=" + std::to_string( s.module_cache_hit ) );
+        " cache_hit=" + std::to_string( s.module_cache_hit ) +
+        " v2_loaded=" + std::to_string( s.module_v2_loaded ) +
+        " v2_baked=" + std::to_string( s.module_v2_baked ) );
     WriteLog(
         "  scm fallback: " + format_seconds( s.scm_fallback_ms ) +
         " count=" + std::to_string( s.scm_fallback ) );