render3d: the water reflection runs only when mirrored water can be on screen; an actor census

The reflection pass is a second copy of the terrain, the vegetation and the actors, and it ran on
every frame of a map with water - looking straight down at a meadow included. The mirrored body is cut
into rectangles where the ground lies below it (32 m over the height map in 8 x 8 blocks, the sea
beyond it coarse), and the pass runs when one meets the view frustum and its water is not all dry
where it shows, outside the frame or behind the ground. Three wrong turns on the way, each kept in a
comment: bounding spheres (a 156 m cell reached into the view from behind the camera), sight lines
that never asked whether the point was in the frame, and a walk of every rectangle every frame (0.1 s
per 400 frames on the PC until the blocks). Built once in 6.6 ms.

Mac, the five shot viewpoints: view c (the meadow) 225 reflection draws -> none; a, b, d, e unchanged;
OpenGL frames byte-identical; self-tests 59/59 on OpenGL and Vulkan; MoltenVK validation adds nothing.
PC camp (lake in view): 1882 draws either way, 3.8 s for 400 frames either way, three runs each,
self-tests 59/59, validation 0. R3D_REFL_ALWAYS=1 runs the pass every frame; R3D_REFL_DBG=1 prints the
test once.

R3D_ACTOR_CENSUS=<frame> prints the lit pass's actors grouped by model: town is 17 models and 69 draws,
and only four rigid models repeat (17 actors) - too little for instancing to be worth a shader variant.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/ludic.render3d/render.ludic

@@ -197,7 +197,7 @@ function r3d_frame(time: int) -> void {
   prof_cpu_mark("streaming")
   if not r3d_no_shadow { prof_begin("shadow");  shadow_pass();  prof_end() }
   prof_cpu_mark("shadow pass")
-  if water_on and not r3d_no_refl { prof_begin("water reflection");  water_reflection_pass();  prof_end() }
+  if water_on and not r3d_no_refl and water_reflect_visible() { prof_begin("water reflection");  water_reflection_pass();  prof_end() }
   prof_cpu_mark("reflection")
   post_begin_scene()
   # the near tree foliage lays its depth down before anything is shaded, so the terrain