fix(client): entity lerp, fog shader safety, type fixes

Entity renderer: add framerate-independent position lerping so entities
slide between tiles instead of snapping. Tuned for Sprint snappiness
and Walk/Careful fluidity.

Fog shader: set ColorRect to transparent fallback so a shader load
failure doesn't paint solid white over the world. Track camera position
(not player position) so fog stays synced during smooth camera pan.
Restructure GLSL to avoid early return (some GPU drivers miscompile it).

Minor type fixes: typed Array[Vector2] in cursor_renderer tick drawing,
untyped Array in inventory_grid to avoid Godot typed-array cast issues.

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

client/ui/inventory_grid.gd

@@ -20,7 +20,7 @@ const SLOT_TEXT := Color("#c8d0e0")
 const SLOT_TEXT_DIM := Color("#8b8ba0")
 const HOTKEY_SIZE := 10
 
-var _slots: Array[Dictionary] = []  # [{item_id, name, slot}] from GameState
+var _slots: Array = []  # [{item_id, name, slot}] from GameState
 var _slot_nodes: Array[Control] = []
 var _selected_slot: int = -1