Reduce from 5-layer to 3-layer fog: clear (forward cone), explored
(light overlay preserving art), and unexplored (solid near-black).
Remove peripheral sector handling from fog_state.gd.
Also preserve exploration data across texture resizes — previously,
resizing the fog texture lost all explored-tile state, causing tiles
behind the player to render as unexplored black instead of light fog.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Define VIS_HIDDEN/PERIPHERAL/FORWARD and EXP_UNEXPLORED/EXPLORED/
VISIBLE in FogState. Replaces 7 magic number usages in _resize() and
update_from_state(). Provides stable assertion targets for fog tests.
Implements #476.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>