Improve selected-NPC ring visuals, anchoring, and occlusion behavior

Selection ring rendering has been upgraded to better match WoW-like target feedback and remain readable across complex geometry.

Visual updates:
- Reworked ring mesh/shader from a simple two-band strip to a unit-disc + radial fragment shaping.
- Implemented a thinner outer ring profile.
- Added an inward color/alpha gradient that fades from the ring toward the center.

Placement/anchoring updates:
- Added CharacterRenderer::getInstanceFootZ() to query model foot plane from instance bounds.
- Added Application::getRenderFootZForGuid() to resolve per-GUID foot height via live instance mapping.
- Updated GameScreen target selection placement to anchor the effect at target foot Z.

Ground/surface stability:
- In renderSelectionCircle(), added floor clamping against terrain, WMO, and M2 floor probes at target XY.
- Raised final placement offset to reduce residual clipping on uneven surfaces.

Depth/visibility behavior:
- Added polygon offset during ring draw to reduce z-fighting.
- Disabled depth testing for the selection effect draw pass (with state restore) so the ring remains visible through terrain/WMO occluders, per requested behavior.

State safety:
- Restored modified GL state after selection pass (depth test / polygon offset / depth mask / cull).

Build validation:
- Verified with cmake build target "wowee" after each stage; final build succeeds.

src/ui/game_screen.cpp

@@ -404,6 +404,10 @@ void GameScreen::render(game::GameHandler& gameHandler) {
             if (target) {
                 targetGLPos = core::coords::canonicalToRender(
                     glm::vec3(target->getX(), target->getY(), target->getZ()));
+                float footZ = 0.0f;
+                if (core::Application::getInstance().getRenderFootZForGuid(target->getGuid(), footZ)) {
+                    targetGLPos.z = footZ;
+                }
                 renderer->setTargetPosition(&targetGLPos);
 
                 // Selection circle color: WoW-canonical level-based colors