feat(render3d): the lake answers to a body standing in it

water.frag takes u_wade - a point and a strength - and puts spreading rings and a
patch of churn into the surface normals there, so a wader marks the water and a
swimmer works the whole of it. It is one uniform in a fragment stage that was
already running, applied after the distance flattening so a disturbance close to
the camera survives it, and it measures no frame cost at all.

Also records the two renderer features that shipped without a changeset.

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

changes/contact-darkening.md

@@ -0,0 +1,9 @@
+bump: patch
+type: feat
+Things sit ON the ground rather than hovering over it. The screen-space GI pass takes
+a second, much tighter set of taps (a 0.40 m radius that grows with distance, with a
+range check so a far surface behind a near one cannot darken it) and folds the result
+into the ambient occlusion it already had. The wide radius answers "how enclosed is
+this", which a trunk meeting grass barely registers; the tight one answers "is
+something touching here", which is the shadow the eye looks for to place an object.
+It is eight taps on a buffer the pass had already bound.

changes/the-world-knows-you-are-there.md

@@ -0,0 +1,9 @@
+bump: minor
+type: feat
+The world answers to a body standing in it. `grass_push(x, z, r)` bends every blade
+within a radius away from a point and presses it down, so a walker opens a bow wave
+through a meadow instead of passing through it; `water_wade(x, z, strength)` puts
+spreading rings and a patch of churn into the lake's normals at a point, so a wader
+marks the surface and a swimmer disturbs the whole of it. Both are one uniform each
+(`u_push`, `u_wade`) read in the vertex and fragment stage that was already running -
+no extra pass, no extra draw.