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/auth/pin_auth.cpp

@@ -63,7 +63,9 @@ static std::vector<uint8_t> randomizePinDigits(const std::string& pinDigits,
         if (idx == 0xFF) {
             throw std::runtime_error("PIN digit not found in remapped grid");
         }
-        out.push_back(static_cast<uint8_t>(idx + 0x30)); // ASCII '0'+idx
+        // PIN grid encodes each digit as its ASCII character ('0'..'9') for the
+        // server-side HMAC computation — this matches Blizzard's auth protocol.
+        out.push_back(static_cast<uint8_t>(idx + '0'));
     }
 
     return out;