Add binary scenery format (eu7 v3) with async baking, replacing SBT

Text scenery component files (.scn/.inc/.scm/.ctr) are compiled into
per-file binary twins (.scnb/.incb/.scmb/.ctrb), handled transparently
at the cParser layer: a fresh twin (mtime-checked, version-matched) is
replayed instead of re-tokenizing text; otherwise the text is parsed and
a twin is compiled alongside it for next time.

Format details:
- per-file string interning: keywords/paths stored once, referenced by
  varint index (so node/endmodel/... are not repeated as text)
- numeric tokens stored as 8-byte IEEE doubles, not ASCII
- includes kept as references with parameters; random sets stored verbatim
  and re-evaluated on every load (choice not frozen at compile time)
- twin writing offloaded to a bounded thread pool so baking overlaps
  scene construction instead of blocking the load

The legacy terrain-only .sbt path is removed; terrain now loads as
ordinary scenery content.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CMakeLists.txt

@@ -149,6 +149,7 @@ set(SOURCES
 "input/messaging.cpp"
 "scene/scene.cpp"
 "scene/scenenode.cpp"
+"scene/scenerybinary.cpp"
 "simulation/simulation.cpp"
 "model/vertex.cpp"
 "audio/audio.cpp"