feat(editor): add --gen-mesh-gravel-pile rubble heap

71st procedural mesh primitive. Hash-distributed pile of
stone cubes in a roughly conical heap. Each stone gets:

  • polar position (radial, theta) with sqrt(rand) on radial
    so stones aren't bunched at center
  • height limited by yMax = pileH * (1 - radial/baseR), so
    larger / more numerous stones land near the base and
    smaller ones perch on top — natural gravel-pile profile
  • size in the 40-100% range of maxStoneSize

The second multi-box "scene" composite primitive (after
--gen-mesh-crate-stack), but using irregular hashed
placement instead of a regular N×M×K grid. Deterministic
from seed: re-running with same args reproduces the
identical pile.

Useful for mine entrances, construction sites, quarries,
ruined walls, abandoned-fort rubble, pirate-cove stash
mounds. Default 24 stones at 0.6 m base radius gives a
reasonable medium-pile.

Uses every shared helper introduced this batch (printWomWrote,
printWomMeshStats, setCenteredBoundsXZ, addFlatBox,
saveWomOrError, parseOpt*, stripExt).

tools/editor/cli_help.cpp

@@ -272,6 +272,8 @@ void printUsage(const char* argv0) {
     std::printf("                         Blacksmith forge: stone hearth + smaller hood + optional chimney (smithy / armorer set dressing)\n");
     std::printf("  --gen-mesh-archery-target <wom-base> [faceR] [faceT] [sides] [postH] [postW] [beamT]\n");
     std::printf("                         Archery target: round face cylinder on a 2-post stand with cross-beam (training yard / fair scene)\n");
+    std::printf("  --gen-mesh-gravel-pile <wom-base> [stoneCount] [baseR] [pileH] [maxStoneSize] [seed]\n");
+    std::printf("                         Gravel pile: hash-distributed stone cubes in a conical heap (mining / construction / rubble)\n");
     std::printf("  --gen-camp-pack <outDir>\n");
     std::printf("                         Convenience: emit tent + firepit + bedroll + canopy + woodpile + haystack into outDir as 6 .wom files\n");
     std::printf("  --gen-blacksmith-pack <outDir>\n");