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>

utilities/utilities.h

@@ -348,6 +348,15 @@ glm::dvec3 LoadPoint(class cParser &Input);
 // extracts a group of tokens from provided data stream
 std::string deserialize_random_set(cParser &Input, char const *Break = "\n\r\t ;");
 
+// like deserialize_random_set, but additionally records every raw token it consumes
+// (including any random-set brackets) into Raw, so the same expression can be replayed
+// and re-randomized later. returns the randomly chosen entry, as deserialize_random_set.
+std::string deserialize_random_set_capture(cParser &Input, std::vector<std::string> &Raw, char const *Break = "\n\r\t ;");
+
+// re-evaluates a raw random-set expression captured by deserialize_random_set_capture,
+// returning a fresh random choice. Pos is advanced through Raw.
+std::string resolve_random_set(std::vector<std::string> const &Raw, std::size_t &Pos);
+
 // extracts a group of <key, value> pairs from provided data stream
 // NOTE: expects no more than single pair per line
 template <typename MapType_> void deserialize_map(MapType_ &Map, cParser &Input)