Removing slopes no longer leaves unowned seal cells; loading clears them

A slope's seal can sit in a cell its neighbour leaves open (a slope's
open corner, or under a grate, screw or screen), but removal only
cleared it when the neighbour was a sloped wall, so cutting a sloped
belt next to a sloped filter or grate could leave a one-cell belt that
nothing owned and nothing could remove. Removal now asks whether the
neighbour fills that cell itself, and loading a world clears belt and
screen cells that no block or seal owns.

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.
 
+## 2026-10-27: Unremovable one-cell belt left after cutting slopes (Claude Code)
+
+- The owner's exported save had a lone belt cell no block owned, so nothing could build there or
+  remove it. It was a sloped belt's seal: a seal may sit in a cell its neighbour leaves open (a
+  slope's open corner, or under a grate, screw or screen), but removal only let it go when the
+  neighbour was a sloped wall. Now `blockOwnsCell` decides (`src/sim/blocks.js`), and loading a
+  world clears belt and screen cells no block or seal owns (`clearStraySeals`, `save.js`).
+- Found with a fuzz of random slope/belt/filter/wall/grate builds removed in random orders and by
+  cut (0 strays in 4000 runs after the fix). The owner's save loads clean and the spot is
+  buildable. Tests in `t_slope_belts.js` fail on the old code. `run_all --browser`: ALL PASSED.
+
 ## 2026-10-27: t42 feedback: downhill belts drop the sidewall too; new tasks (Claude Code)
 
 - Downhill sloped belts now draw as plain belts: no far wall and no ribs, placed or as ghosts or