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/scripts/rendering/fog_shader.gd

@@ -43,7 +43,8 @@ func update_fog() -> void:
 
 	# 2. Position ColorRect to cover the current viewport
 	var vp_size := get_viewport().get_visible_rect().size
-	var zoom := Vector2(2.0, 2.0)  # Must match Camera2D zoom
+	var cam := get_viewport().get_camera_2d()
+	var zoom := cam.zoom if cam else Vector2(2.0, 2.0)
 	var camera_pos := GameState.player_position * TILE_SIZE
 	var half_view := vp_size / (2.0 * zoom)
 	_fog_rect.position = camera_pos - half_view