Cooled magma is ordinary rock: visible in the smooth look and drillable

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

docs/DESIGN.md

@@ -517,6 +517,10 @@ above, including:
   and warned about before it happens (a glow near the limit). A heat-resistant screw comes later
   as an upgrade. Prompted by a long vertical screw that seemed to leak for no reason while it
   carried hot rubble.
+- **Melts that cool loose are ordinary rock (2026-10-27).** Magma poured from the bag became an
+  invisible, undrillable basalt the game treated as built. Now any melt except metal and glass
+  cools into natural rock wherever it came from, and any solid no block, mold or machine owns can
+  be drilled, so nothing can get permanently stuck in the world.
 
 ## Ideas not built yet
 

docs/agent-log.md

@@ -4,6 +4,19 @@ 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: cooled magma visible and drillable (Claude Code)
+
+- Owner report: bag magma in a brick box left invisible basalt that couldn't be picked up,
+  drilled or built on; melted screws left similar bits. Cause: melts froze into non-natural
+  rock unless the liquid was natural, the drill only breaks natural rock, and the smooth look's
+  terrain field skipped non-natural solids.
+- `src/sim/heat.js`: non-metal, non-glass melts freeze into natural rock; bag water into natural
+  ice. `src/sim/drill.js`: `crumbly` no longer needs F_NAT; `builtCell` protects slope corner cells.
+  `src/render/world.js`: the terrain field covers any terrain-class solid without a block or
+  machine. Note: the bot still passes through old stuck basalt until it is drilled (it only
+  collides with natural solids).
+- Tests: 7 checks in `test/t_drill.js`; `python3 test/run_all.py --browser` all passed.
+
 ## 2026-10-27: t67 phase 4, "doesn't connect" marks (Claude Code)
 
 - `nearMiss(bx, by, end, planned)` in `src/sim/joins.js`: an end with no join that would join