refactor: extract color write mask, name frustum epsilon, add comments

- vk_pipeline: extract kColorWriteAll constant from 4 duplicated RGBA
  bitmask expressions across blend mode functions, with why-comment
- frustum: name kMinNormalLenSq epsilon (1e-8) with why-comment —
  prevents division by zero on degenerate planes
- dbc_loader: add why-comment on DBC field width validation — all
  fields are fixed 4-byte uint32 per format spec
- pin_auth: replace 0x30 hex literal with '0' char constant, add
  why-comment on ASCII encoding for server HMAC compatibility

src/rendering/frustum.cpp

@@ -64,7 +64,10 @@ void Frustum::extractFromMatrix(const glm::mat4& vp) {
 
 void Frustum::normalizePlane(Plane& plane) {
     float lenSq = glm::dot(plane.normal, plane.normal);
-    if (lenSq > 0.00000001f) {
+    // Skip normalization for degenerate planes (near-zero normal) to avoid
+    // division by zero or amplifying floating-point noise into huge normals.
+    constexpr float kMinNormalLenSq = 1e-8f;
+    if (lenSq > kMinNormalLenSq) {
         float invLen = glm::inversesqrt(lenSq);
         plane.normal *= invLen;
         plane.distance *= invLen;