render3d: the uniform ring is 16 MB a half and grows when a frame outgrows it; streamed layers are sized for the fog's reach

The ring held 2 x 64 MB of host memory for a frame that uses 3 MB at most (2761 draws at the
overlook, 1.7 MB in town): it starts at 16 MB a half, and a frame that ever runs out grows it for the
frames after (gvk_ring_grow, making the kept sets again; R3D_RING_KB=<n> starts small to watch it).
-95 MB at every fog level, off included.

A streamed layer's arrays and its stream's arena are made for the reach a fog wall leaves (twice its
area's share, at least 4096 instances) and emptied to refill within the frame budget
(fog_streams.ludic). The near streams' reach is already inside most walls, so it is -9 MB at 30 m.

R3D_VKMEM adds the ring's high-water mark, the largest buffers and the streamed layers' share.
Footprint at the overlook: 2008 MB off (2108 before this phase), 1748 MB at 175 m, 1682 MB at 30 m.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.render3d/r3d.ludic

@@ -38,6 +38,7 @@ import "fog_casters.ludic"
 import "fog_impostors.ludic"
 import "actor.ludic"
 import "stream.ludic"
+import "fog_streams.ludic"
 import "grass.ludic"
 import "grass_gpu.ludic"
 import "water.ludic"