Delete the JRPG's collision too, and wall the map with proxies

cell_solid and feet_blocked go, and so does the prediction in the player's
movementlogicfunc. That prediction was only ever exact because the town has zero
gravity and zero drag -- v is t, so `x + tx * dt` is where the step lands. A map
with gravity would have had to fold ey in as well, which is the game
re-implementing the integrator. Resolution runs after the move now, so there is
nothing to predict.

The map's decoration layer carries `collidable`, which retires JRPG_LAYER_SOLID:
akgl_TilemapLayer has no name member, so the game and the map had to agree on an
index out of band and inserting a layer in Tiled broke it.

The edge of the world is not on any layer, so it is four static proxies covering
the outer ring plus a tile of overhang -- one pool slot per side instead of a
hundred solid tiles, and the first use of the static-proxy path in either
example. They carry LAYER_STATIC explicitly: shape_box defaults a shape to
LAYER_ACTOR, and a wall left on that layer is a wall everything walks through.

Only the player gets a shape. NPCs stand still and are spoken to; the follower
is a child snapped to its parent every step.

Verified against the old implementation with the demo script rewritten to hold
one direction: holding left into a building stops the player at x=122 with both,
and holding up into the map's edge stops them at y=-4 with both.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBDV2fqgnUAcqCKqKvc71

docs/20-tutorial-sidescroller.md

@@ -1,14 +1,14 @@
 # 20. Tutorial: a 2D sidescroller
 
 A complete game: a Tiled level, a player who runs and jumps, platforms that hold him up,
-four coins to collect, a blob that patrols and a moth that flies. It is about nine hundred
-lines of C — a header and four translation units, and rather more comment than that — and it
-builds and runs as part of this repository's ordinary `ctest` run.
+four coins to collect, a blob that patrols and a moth that flies. It is a header and three
+translation units, rather more comment than code, and it builds and runs as part of this
+repository's ordinary `ctest` run.
 
-It is here for one reason above the others. **libakgl has no collision detection at all**,
-and a sidescroller is the shortest path to finding that out. So this chapter is mostly about
-what you write when the engine stops, and where exactly that code has to live for the frame
-to come out right.
+It used to be four translation units. The fourth was 383 lines of collision, and it is gone
+— which makes this chapter as much about **where in the frame a thing has to happen** as
+about what to write. That is the lesson that survived: the same test, run a few lines
+earlier, is wrong in ways that take an afternoon to diagnose.
 
 The program is `examples/sidescroller/`; the art, the map and the JSON are in
 `docs/tutorials/assets/sidescroller/`. Every listing below is quoted straight out of those