feat(senses): walkers slide round still things, and lines of sight are the world's to answer

- ludic.wildlife: WildlifeWorld.push / pushed_x / pushed_z (defaults change nothing); a ground
  walker's step is slid clear of a still thing with a body from its species' size and scale, and
  a step that gets nowhere turns away as the water does. Birds are not pushed; no dice added.
- ludic.npc: NpcWorld.push / pushed_x / pushed_z (a person 0.35 m round) in npc_walk, and
  NpcWorld.clear (default true), which npc_line_ok asks at 1 m so a detour does not cut a trunk.
- ludic.aim: AimView.clear replaces AimView.ground and the ground march; the pick asks the line
  a body's width short of where the ray enters the thing (aim_clear_at(t, back)), so a solid
  thing does not hide itself. probe.ludic (the ground-agreement probe) and AimView.dry are gone.
- ludic.photo: PhotoLens.clear replaces PhotoLens.ground, PhotoLens.trunk and the march.

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

packages/ludic.photo/README.md

@@ -36,14 +36,14 @@ export port PhotoLens {
   x, y, z, fx, fy, fz, fov,                       # the camera
   project(x, y, z) -> bool, sx(), sy(),           # into the picture, 0..1
   visible(x, y, z, r) -> bool,                    # the frustum
-  ground(x, z), trunk(x, z, r) -> bool,           # what a line of sight walks
+  clear(ax, ay, az, bx, by, bz) -> bool,          # a line of sight: the ground and every still thing
   range(), steady() -> bool                       # the lens in the pack
 }
 export port PhotoLight { hour, night, moon, dim, bonus, sun_x, sun_z }
 export port PhotoRules { behaviour, subject_value(sp), legend_value(sp), mult }
 ```
 
-Every member has a default: a lens at the origin looking down -z on flat open ground at noon.
+Every member has a default: a lens at the origin looking down -z with nothing in the way at noon.
 
 ## API