feat: better walls

AGENTS.md

@@ -61,8 +61,9 @@ rules live in `.agents/rules/*.md`.
   - `main.ts` — game loop, input, preset switching, canvas blit, FPS meter.
   - `assets.ts` — load `/assets/*.png` → `Texture` (zero-copy; ImageData bytes
     are already the `Color` layout).
-  - `level.ts` — builds the playground: a flat stone-floored room (three walls,
-    north side open) in the center of a big grassy `Terrain` world (~20x across).
+  - `level.ts` — builds the playground: a flat stone-floored room (three thick
+    walls via `slab`, north side open) in the center of a big grassy `Terrain`
+    world (~20x across).
     Per-texture meshes incl. the outdoor grass `ground`, `Aabb` colliders, NPC
     position, `Terrain` config + `GROUND_DIVISIONS`/`GROUND_UV`, sky/cloud config.
     The stone floor is lifted by `FLOOR_LIFT` (a z-bias) so it stays clean over the
@@ -70,8 +71,10 @@ rules live in `.agents/rules/*.md`.
     quads -- no subdivision needed since texturing is perspective-correct; ground
     triangle count is `GROUND_DIVISIONS` (fixed grid, so world size sets cell
     chunkiness, not tri count).
-  - `player.ts` — feet-cylinder player: gravity/jump + circle-vs-AABB/-circle
-    collision; ground height comes from `Terrain.height` (plus standable AABBs).
+  - `player.ts` — feet-cylinder player: gravity/jump + Shift-run
+    (`RUN_MULTIPLIER`) + circle-vs-AABB/-circle collision, substepped so fast
+    running can't tunnel walls; ground height from `Terrain.height` (plus
+    standable AABBs).
 - `index.html` — Vite entry at repo root; holds the `#screen` canvas and the
   `#fps` meter div (styled inline).
 - `scripts/gen-assets.ts` — procedurally draws the placeholder textures and
@@ -130,9 +133,10 @@ branching in the cloud shader. Cost scales with sky resolution — fine at
 
 ## Controls
 
-WASD move · mouse look (click canvas to pointer-lock) · **Space** jump ·
-**1/2/3** switch look presets. FPS shown bottom-right. The room's north wall is
-open — walk out onto the terrain and toward the peaks.
+WASD move · **Shift** run (speed ×`RUN_MULTIPLIER` in `app/player.ts`) · mouse
+look (click canvas to pointer-lock) · **Space** jump · **1/2/3** switch look
+presets. FPS shown bottom-right. The room's north wall is open — walk out onto
+the terrain and toward the peaks.
 
 ## Code style (oxlint-enforced — match it)