Sloped walls with a flat 45° face, and diagonal wall lines (t36)

- New Sloped wall piece: a half-block triangle in any wall material, at half a wall's cost.
  Dragging walls or slopes at about 45° lays a diagonal line; the drag picks the slope's
  orientation and T flips or turns it.
- Each slope fills one cell past its corner, so a diagonal run is sealed against grains and water.
- New test/t_slopes.js: a slope funnel passes 400/400 sand (square steps 379/400).

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

docs/agent-log.md

@@ -4,6 +4,17 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-25: t36: sloped walls and diagonal wall lines (Claude Code, Codex)
+
+- `BP_SLOPE_BL`..`BP_SLOPE_TR` (15-18) in `src/sim/pieces.js` (`isSlope`, `isWall`, `slopeCell`,
+  `slopeCode`); `src/sim/blocks.js`: half cost and refund, `slopeSeal` corner cell and
+  `slopeSealOwner`, `diagonalDrag` and `blockLine(.., diagonal)`. Build menu, T key
+  (`turnSlope`), previews, copy/paste, undo, blueprints, hover, both looks.
+- `test/t_slopes.js`: funnel 400/400 sand (square steps 379/400), water 400/400, no leaks in all
+  four orientations from both sides, bars slide, every wall material; play.py menu and diagonal
+  drag. Seeds unchanged. `run_all.py --browser` ALL PASSED. Screenshots
+  `shots/t36-funnel-*.png`, scene `tools/scenes/t36-funnel.js`.
+
 ## 2026-10-25: t33: loose grains and solid ground look different (Claude Code, Codex)
 
 - Smooth look (`src/render/smooth-shaders.js`): natural ground gets a soft seam where it meets