render3d: actors cast only into the cascades they can shade; Vulkan GPU timings and a mean frame split in R3D_PROF

Where the frame goes, before optimising it further. R3D_PROF now prints the average frame: CPU before
the swap (the game's share apart), GPU and swap, and the renderer's CPU phases in frame order - the
shadow pass split into cascade fit, scatter casters and actor casters. On Vulkan the per-pass GPU
table comes from timestamp queries (host query reset asked for where the device has it); MoltenVK's
attribution is tile-based and not to be trusted per pass, the PC's is.

What it showed on the PC (camp): 4.3 ms CPU and 5.3 ms GPU a frame; the shadow pass was the largest
CPU phase (1.8 ms) and actors half of that. Every actor within 300 m was drawn into all five cascades,
though the outer two only shade receivers from 212 and 935 m out: 160 actors and 300 draws into each.
cast_band_reaches - the flowers' reach test, now shared - skips an actor for a cascade it cannot shade
(receivers counted from 0.85 of the previous split, where sunShadow's cross-fade begins).

PC camp, two runs each: mean frame 9553/9601 -> 8664/8656 us; CPU 4.3 -> 3.7 ms; shadow GPU 1.14 ->
0.79 ms; actor-shadow CPU 1.02 -> 0.60 ms; 2039 -> 1411 draws; self-tests 59/59, validation 0.
Mac: OpenGL shot viewpoints and the camp byte-identical; town 19 px at <= 2/255 on two flower stems a
few metres from the camera - the accepted leftover-binding difference, no shadow; self-tests 59/59 on
OpenGL and Vulkan. R3D_CAST_ALL=1 draws every caster into every cascade.

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

packages/ludic.render3d/shadow.ludic

@@ -156,6 +156,7 @@ function shadow_pass() -> void {
   for c in 0 .. SHADOW_CASCADES {
     sh_cascade = c
     shadow_fit(c, near, sh_split[c])
+    prof_cpu_mark("shadow fit")
     near = sh_split[c]
     gpu_fb_depth_layer(sh_tex, c)
     if r3d_debug and c == 0 { let st = gpu_fb_status();  print(`shadow fbo status {st}`) }