feat(rendering): GPU architecture + visual quality fixes

M2 GPU instancing
- M2InstanceGPU SSBO (96 B/entry, double-buffered, 16384 max)
- Group opaque instances by (modelId, LOD); single vkCmdDrawIndexed per group
- boneBase field indexes into mega bone SSBO via gl_InstanceIndex

Indirect terrain drawing
- 24 MB mega index buffer (6M uint32) + 64 MB mega vertex buffer
- CPU builds VkDrawIndexedIndirectCommand per visible chunk
- Single VB/IB bind per frame; shadow pass reuses mega buffers
- Replaced vkCmdDrawIndexedIndirect with direct vkCmdDrawIndexed to fix
  host-mapped buffer race condition that caused terrain flickering

GPU frustum culling (compute shader)
- m2_cull.comp.glsl: 64-thread workgroups, sphere-vs-6-planes + distance cull
- CullInstanceGPU SSBO input, uint visibility[] output, double-buffered
- dispatchCullCompute() runs before main pass via render graph node

Consolidated bone matrix SSBOs
- 16 MB double-buffered mega bone SSBO (2048 instances × 128 bones)
- Eliminated per-instance descriptor sets; one megaBoneSet_ per frame
- prepareRender() packs bone matrices consecutively into current frame slot

Render graph / frame graph
- RenderGraph: RGResource handles, RGPass nodes, Kahn topological sort
- Automatic VkImageMemoryBarrier/VkBufferMemoryBarrier between passes
- Passes: minimap_composite, worldmap_composite, preview_composite,
  shadow_pass, reflection_pass, compute_cull
- beginFrame() uses buildFrameGraph() + renderGraph_->execute(cmd)

Pipeline derivatives
- PipelineBuilder::setFlags/setBasePipeline for VK_PIPELINE_CREATE_DERIVATIVE_BIT
- M2 opaque = base; alphaTest/alpha/additive are derivatives
- Applied to terrain (wireframe) and WMO (alpha-test) renderers

Rendering bug fixes:
- fix(shadow): compute lightSpaceMatrix before updatePerFrameUBO to eliminate
  one-frame lag that caused shadow trails and flicker on moving objects
- fix(shadow): scale depth bias with shadowDistance_ instead of hardcoded 0.8f
  to prevent acne at close range and gaps at far range
- fix(visibility): WMO group distance threshold 500u → 1200u to match terrain
  view distance; buildings were disappearing on the horizon
- fix(precision): camera near plane 0.05 → 0.5 (ratio 600K:1 → 60K:1),
  eliminating Z-fighting and improving frustum plane extraction stability
- fix(streaming): terrain load radius 4 → 6 tiles (~2133u → ~3200u) to exceed
  M2 render distance (2800u) and eliminate pop-in when camera turns;
  unload radius 7 → 9; spawn radius 3 → 4
- fix(visibility): ground-detail M2 distance multiplier 0.75 → 0.9 to reduce
  early pop of grass and debris

include/rendering/terrain_renderer.hpp

@@ -60,6 +60,11 @@ struct TerrainChunkGPU {
     float boundingSphereRadius = 0.0f;
     glm::vec3 boundingSphereCenter = glm::vec3(0.0f);
 
+    // Phase 2.2: Offsets into mega buffers for indirect drawing (-1 = not in mega buffer)
+    int32_t megaBaseVertex = -1;
+    uint32_t megaFirstIndex = 0;
+    uint32_t vertexCount = 0;
+
     bool isValid() const { return vertexBuffer != VK_NULL_HANDLE && indexBuffer != VK_NULL_HANDLE; }
 };
 
@@ -200,6 +205,25 @@ private:
     bool fogEnabled = true;
     int renderedChunks = 0;
     int culledChunks = 0;
+
+    // Phase 2.2: Mega vertex/index buffers for indirect drawing
+    // All terrain chunks share a single VB + IB, eliminating per-chunk rebinds.
+    // Indirect draw commands are built CPU-side each frame for visible chunks.
+    VkBuffer megaVB_ = VK_NULL_HANDLE;
+    VmaAllocation megaVBAlloc_ = VK_NULL_HANDLE;
+    void* megaVBMapped_ = nullptr;
+    VkBuffer megaIB_ = VK_NULL_HANDLE;
+    VmaAllocation megaIBAlloc_ = VK_NULL_HANDLE;
+    void* megaIBMapped_ = nullptr;
+    uint32_t megaVBUsed_ = 0;  // vertices used
+    uint32_t megaIBUsed_ = 0;  // indices used
+    static constexpr uint32_t MEGA_VB_MAX_VERTS   = 1536 * 1024; // ~1.5M verts × 44B ≈ 64MB
+    static constexpr uint32_t MEGA_IB_MAX_INDICES  = 6 * 1024 * 1024; // 6M indices × 4B = 24MB
+
+    VkBuffer indirectBuffer_ = VK_NULL_HANDLE;
+    VmaAllocation indirectAlloc_ = VK_NULL_HANDLE;
+    void* indirectMapped_ = nullptr;
+    static constexpr uint32_t MAX_INDIRECT_DRAWS = 8192;
 };
 
 } // namespace rendering