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/pipeline/dbc_loader.cpp

@@ -64,7 +64,8 @@ bool DBCFile::load(const std::vector<uint8_t>& dbcData) {
         return false;
     }
 
-    // Validate record size matches field count
+    // DBC fields are fixed-width uint32 (4 bytes each); record size must match.
+    // Mismatches indicate a corrupted header or unsupported DBC variant.
     if (recordSize != fieldCount * 4) {
         LOG_WARNING("DBC record size mismatch: recordSize=", recordSize,
                     " but fieldCount*4=", fieldCount * 4);