refactor(ui): add stance icons to implant badge (#787)

Replace text-only stance labels with SVG icons + tint shader per D-086.
Alpha-mask shader swaps color at runtime via ShaderMaterial parameter.
Icons: walk, crouch, careful, sprint at 18px inside ImplantPanel row.
Also fix stale UILayer/HUD test path in test_ui_framework_sprint15.

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

client/shaders/icon_tint.gdshader

@@ -0,0 +1,10 @@
+shader_type canvas_item;
+// D-086: Runtime icon tint — replaces color channel with tint_color, preserves alpha mask.
+// Applied to SVG-sourced TextureRect nodes so stance/status icons can be color-coded at runtime.
+// Default: #c8d0e0 (insert chrome) — identical to the SVG's authored stroke color.
+uniform vec4 tint_color : source_color = vec4(0.784, 0.816, 0.878, 1.0);
+
+void fragment() {
+	float a = texture(TEXTURE, UV).a;
+	COLOR = vec4(tint_color.rgb, a * tint_color.a);
+}