Fog of war: underground is black until the bot has seen it (t8)

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

docs/agent-log.md

@@ -4,6 +4,22 @@ 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.
 
+## 2026-10-24: t8 Fog of war for unexplored areas (Claude Code, Codex)
+
+- Decided with the owner: revealed by line of sight; unseen is black with a soft edge.
+- New `src/sim/fog.js` (2×2-grain cells, 0-255, only ever rises): `fogInit` (sky and surface
+  seen), `fogLook` (256 rays, 48 grains, through air, gas and liquid, 4 grains into rock), called
+  every 4 ticks from `simTick`. Never read by the simulation.
+- `src/sim/save.js`: fog saved; saves without it rebuild it (surface, 12 grains around built
+  things, the bot's view). `src/sim/world.js`, `worldgen.js`: allocate and initialize.
+- `src/render/world.js`: grains scaled by a bilinear fog value; fully unseen grains drawn like
+  out-of-world grains; no glow from fog; thermal lens fogged too. `overlays.js`: rain and
+  particles hidden in fog. `src/ui/info.js`: "Unexplored". Setting `fog` (Game tab).
+- Browser tests and tools turn fog off (`play.py`, `t_smooth.py`, `shot.py`, `perf.py`,
+  `playthrough.py`) except the fog checks. New `test/t_fog.js`; `docs/systems/fog.md`.
+- Tests: all 12 headless suites, `play.py` (185 checks), `t_smooth` pass. Screenshots checked in
+  both looks. `tools/perf.py`: drawWorld 0.34 → 0.43 ms with fog on (no change with light off).
+
 ## 2026-10-24: t5 Material selection menu (Claude Code, Codex)
 
 - Decided with the owner: F opens it for walls and molds, R keeps cycling.