feat(assets): add 3D-to-2D sprite render pipeline and /render-sprite skill

Godot @tool render scene that takes a textured 3D model and produces
2D sprites at 1024/256/64 from 4 cardinal directions with outlines
applied at working resolution. Camera at "the angle" (-72.5deg ortho).
Includes two POC wall models with Nano Banana textures, CLI wrapper
script with headless import step, and .uid files added to gitignore.

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

client/tooling/sprite_renderer/render_scene.tscn

@@ -0,0 +1,27 @@
+[gd_scene load_steps=2 format=3 uid="uid://cqvfv4uhbgq6h"]
+
+[ext_resource type="Script" path="res://tooling/sprite_renderer/render_export.gd" id="1_render_script"]
+
+[node name="RenderScene" type="Node3D"]
+script = ExtResource("1_render_script")
+
+[node name="SubViewport" type="SubViewport" parent="."]
+transparent_bg = true
+size = Vector2i(1024, 1024)
+render_target_update_mode = 1
+own_world_3d = true
+
+[node name="Camera3D" type="Camera3D" parent="SubViewport"]
+transform = Transform3D(1, 0, 0, 0, 0.309017, 0.951057, 0, -0.951057, 0.309017, 0, 3, 1)
+projection = 1
+size = 1.4
+near = 0.01
+far = 10.0
+
+[node name="DirectionalLight3D" type="DirectionalLight3D" parent="SubViewport"]
+transform = Transform3D(1, 0, 0, 0, 0.309017, 0.951057, 0, -0.951057, 0.309017, 0, 3, 1)
+light_color = Color(1, 1, 1, 1)
+light_energy = 1.0
+shadow_enabled = false
+
+[node name="ModelRoot" type="Node3D" parent="SubViewport"]