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.

include/core/application.hpp

@@ -70,6 +70,7 @@ public:
 
     // Render bounds lookup (for click targeting / selection)
     bool getRenderBoundsForGuid(uint64_t guid, glm::vec3& outCenter, float& outRadius) const;
+    bool getRenderFootZForGuid(uint64_t guid, float& outFootZ) const;
 
     // Character skin composite state (saved at spawn for re-compositing on equipment change)
     const std::string& getBodySkinPath() const { return bodySkinPath_; }