The stylized tree's foliage was a stack of ellipsoid lobes (canopy) or loose
per-tip cards (cards): the first reads as a bunch of grapes, the second clumps
and leaves holes because every card is drawn independently.
Add a cluster foliage mode built the way stylized foliage is normally authored:
* leaf centres on a plane are blue noise (Poisson disk), so the distribution
has no clumps and no bare patches;
* only the leaves emit geometry, which is how a CPU mesh expresses
'leaves opaque, everything else transparent' without alpha sorting or
transparent overdraw;
* 8-10 such planes are placed around the cluster's Y axis with a small random
tilt and offset, plus a few near-horizontal planes that close the poles;
* vertex normals are transferred from the cluster sphere instead of the leaf's
own plane, so a bundle of flat cards still shades as a rounded mass;
* both windings are emitted, because the engine's mesh3d pipelines cull back
faces unless a Material opts into two-sided rasterization.
Cluster centres are picked from the existing foliage anchors in a shuffled order
under a Poisson separation test, so they cover the crown evenly, stay inside a
hard geometry budget, and the twigs visibly enter the foliage.
FoliageCluster is a standalone implementation unit with its own
FoliageClusterDesc so other recipes can reuse it, and its disk sampling goes
through the module's existing Bridson sampler (poissonDiskPoints) instead of a
second blue-noise implementation.
Also fix the mesh.tree schema, which advertised clusters and colonization as
leafMode/branchAlgorithm choices while the generator rejected both, and was
missing canopy.
Tests live in their own translation unit (the shared procgen.cpp is already
~4500 lines): property tests for the sampler, the sphere normal transfer and
two-sided emission, a case asserting every cluster knob is exposed in the
schema and actually changes the mesh, and an opt-in step-by-step render dump
(EVENGINE_TREE_STEPS_DIR) that draws each construction stage.
Verified with ctest -R 'mesh\.|poisson' (136/136).
