Fix File stat-before-open crash risk, JSON string-quoting bug, and MString buffer overflow

- File: stat() only after a successful open() (previously ran first with
  its result ignored, feeding a missing file's garbage st_size into a
  realloc - could crash the whole APIServer on a request for a
  nonexistent static asset)
- JString::setString(): always quote+escape the value instead of going
  through set()'s literal-value guess, which misclassified any string
  starting with digits (e.g. a UUID) as a bare numeric literal and wrote
  it unquoted, corrupting later edits to the same JSON document
- MString::remove() (both overloads): fixed an off-by-one that read one
  byte past the buffer on a completely full string (found via ASan under
  concurrent load)
- CGIFormData: added fields() so callers can iterate every parsed field
  without knowing names in advance (used to merge POST body into
  jet::Global::variables the same way query-string params already are)

JString.cpp

@@ -474,7 +474,11 @@ namespace coreutils {
 	formatted << value;
       else
 	formatted << "\"" << jsonEscape(value).c_str() << "\"";
+      setFormatted(path, formatted);
+   }
 
+   void JString::setFormatted(ZString path, MString formatted) {
+      reset();
       if (locate(path)) {
 	 int startOffset = offset();
 	 skipValue();
@@ -491,8 +495,28 @@ namespace coreutils {
    }
 
    void JString::setString(ZString path, std::string value) {
+      // Deliberately NOT routed through set() (which calls isLiteralValue()
+      // to guess whether the raw text should be quoted) - setString() is
+      // explicitly, unambiguously a string setter, so the value must
+      // ALWAYS be quoted+escaped regardless of what it looks like.
+      // isLiteralValue() classifies anything ZString::startsWithNumber()
+      // accepts as a bare numeric literal - true for "42", but ALSO true
+      // for a string that merely STARTS with digits, like a UUID
+      // ("00146df8-e42c-11e7-9c78-525400b869dc"). Going through set()
+      // wrote that UUID completely unquoted; skipValue() then only
+      // recognizes the JSON-number PREFIX ("00146") as the value's real
+      // span when locating it for a later update, so the next setString()
+      // call spliced its own new key/value pair in right after "00146",
+      // leaving "df8-e42c-...-869dc" as orphaned text outside the object
+      // entirely - malformed JSON, confirmed via a real corrupted session
+      // file (jet-2.0's own native $[@name] session variables, backed by
+      // this class - see Global.h). Every other setString() caller
+      // sitewide was silently exposed to the same risk for any string
+      // value that happens to start with a digit.
       MString v(value);
-      set(path, v);
+      MString formatted;
+      formatted << "\"" << jsonEscape(v).c_str() << "\"";
+      setFormatted(path, formatted);
    }
 
    void JString::setNumber(ZString path, double value) {