Add StreamReader and rebuild Parser/LineParser/IMFHeader/IMFBoundary on it

Introduces StreamReader to accumulate socket data delivered in arbitrary
chunks into one buffer, so a Parser can build an object (a line, a header
block, a MIME boundary) that spans any number of packets without losing
its place. Parser is redesigned around this: no longer inherits MString
(was redundant with its own buffer member), takes a StreamReader&, and
drives a resumable COMPLETE/INCOMPLETE/DATA_AVAIL state-machine protocol
via doParse(). LineParser, IMFHeader, and IMFBoundary are rewritten as
real, working resumable parsers against the shared buffer instead of the
previous non-compiling stubs (mismatched constructors/return types, a
syntax error, checkParse() never implemented).

IMFFormData is left as the stub it already was, just restored to
compiling against the new IMFHeader signature - fully wiring it up needs
IMFMultipart/IMFText migrated to StreamReader too, out of scope here.

Verified with real runtime tests (not just compilation) covering data
split mid-line, mid-CRLF, mid-header-fold, and mid-boundary-marker across
separate feed() calls, plus StreamReader::compact(). Along the way fixed
a header-value bug (space after ':' wasn't skipped) and a double
cursor-offset subtraction bug in two places caused by forgetting that
ZString::getLength() is already cursor-relative, not the raw buffer size.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

StreamReader.cpp

@@ -0,0 +1,27 @@
+#include "StreamReader.h"
+
+namespace coreutils {
+
+   StreamReader::StreamReader() {}
+
+   void StreamReader::feed(ZString &data) {
+      buffer.write(data);
+   }
+
+   MString &StreamReader::getBuffer() {
+      return buffer;
+   }
+
+   void StreamReader::compact() {
+      int parsedLength = buffer.getCursor() - buffer.getData();
+      if (parsedLength > 0) {
+         buffer.remove(0, parsedLength);
+         buffer.setCursor(buffer.getData());
+      }
+   }
+
+   bool StreamReader::hasUnparsedData() {
+      return !buffer.eod();
+   }
+
+}