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_arg_required.cpp

@@ -56,7 +56,7 @@ const char* const kArgRequired[] = {
     "--gen-mesh-watchpost", "--gen-mesh-water-trough",
     "--gen-mesh-training-dummy", "--gen-mesh-hitching-post",
     "--gen-mesh-outhouse", "--gen-mesh-forge",
-    "--gen-mesh-archery-target",
+    "--gen-mesh-archery-target", "--gen-mesh-gravel-pile",
     "--gen-camp-pack", "--gen-blacksmith-pack",
     "--gen-mesh-table", "--gen-mesh-lamppost", "--gen-mesh-bed",
     "--gen-mesh-ladder", "--gen-mesh-well", "--gen-mesh-signpost",