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>

IMFHeader.h

@@ -1,26 +1,48 @@
 #ifndef __IMFHeader_h__
 # define __IMFHeader_h__
 
-# include "StreamReader.h"
 # include "Parser.h"
 # include "MString.h"
-# include "IMFHeaderField.h"
-# include <vector>
+# include <map>
 
 namespace coreutils {
-   
+
+   ///
+   /// IMFHeader parses a full block of IMF/MIME-style headers (RFC 5322
+   /// "name: value" pairs, one per line, with folded continuation lines -
+   /// any line starting with a space or tab is appended to the previous
+   /// field's value) from the shared StreamReader, stopping at the blank
+   /// line that terminates the header block. Resumable across any number of
+   /// StreamReader::feed() calls - partially-read names/values accumulate
+   /// in the name/field members rather than being discarded between calls.
+   ///
+
    class IMFHeader : public Parser {
-      
+
     public:
+
       IMFHeader(StreamReader &reader);
 
-      virtual int parse();
-      
+      ///
+      /// Valid once doParse() has returned Status::COMPLETE - every header
+      /// field parsed, keyed by field name.
+      ///
+
+      std::map<MString, MString> &getHeaders();
+
+    protected:
+
+      Status parse() override;
+
     private:
+
+      enum State { NAME, SKIP_SPACE, FIELD, FOLD_CHECK } state = NAME;
       MString name;
       MString field;
       std::map<MString, MString> headers;
-      
+
+      void commitField();
+
    };
 
 }