fix(client): address PR #22 review — 3 critical bugs, 2 warnings, 8 suggestions

Critical fixes:
- fog_state: guard _compute_bounds() against all-invalid tiles (negative Rect2i crash)
- fog_shader: read Camera2D zoom dynamically instead of hardcoded Vector2(2,2)
- world_radial: set custom_minimum_size in _ready() from spoke geometry

Warnings:
- fog.gdshader: tighten PERIPHERAL_LOW 0.15→0.55 to match D-059 peripheral band
- Extract color_for_entity_kind() to Constants.gd, decouple CursorRenderer from EntityRenderer

Documentation: shallow copy assumption, gradual decay TODO, tween guard rationale,
fade timing rationale, ToggleInsert TODO, monologue consume-once, hover offset safety,
v6 fixture gap TODO.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/shaders/fog.gdshader

@@ -25,7 +25,7 @@ const vec3 DARK_OVERLAY = vec3(0.02, 0.02, 0.05);
 // D-059 thresholds (after bilinear filtering)
 // Forward tiles = 1.0, Peripheral = 0.706 (180/255), not-visible = 0.0
 const float CLEAR_THRESHOLD = 0.85;   // Above this: fully clear
-const float PERIPHERAL_LOW = 0.15;    // Below this: transition to deep/unexplored
+const float PERIPHERAL_LOW = 0.55;    // Below this: transition to deep/unexplored
 
 void fragment() {
     // Map UV (0-1 across ColorRect) to world pixels, then to tile coordinates