feat(client): manual exponential camera smoothing (#117)

Replace Godot built-in Camera2D smoothing with manual lerp using
CAMERA_SMOOTHING_SPEED (8.0) in constants.gd — same exponential
smoothing pattern as entity_renderer.gd. Teleport snap preserved
via _teleport_in_progress flag. D-015 fixed-north camera lock intact.

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

client/scripts/constants.gd

@@ -99,6 +99,11 @@ const DIALOGUE_MAX_WIDTH: int = 1200
 # Default camera zoom — used as fallback when get_camera_2d() returns null
 const CAMERA_DEFAULT_ZOOM: Vector2 = Vector2(2.0, 2.0)
 
+# #117: Camera smoothing speed — exponential interpolation via manual lerp in main.gd.
+# Same pattern as EntityRenderer.LERP_SPEED. At 8.0: ~55% convergence after 0.1s.
+# Slightly softer than entity movement (12.0) for a touch of cinematic camera lag.
+const CAMERA_SMOOTHING_SPEED: float = 8.0
+
 # #517: Implant UI font color grading — avoid pure white, project through a lens
 const IMPLANT_TEXT_COLOR: Color = Color("#E0F7FA")   # Cyan-white — primary text
 const IMPLANT_TEXT_DIM: Color = Color("#9EBFC4")     # Dimmed variant — secondary text