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>

IMFBoundary.cpp

@@ -1,30 +1,49 @@
 #include "IMFBoundary.h"
 #include "Exception.h"
-#include <unistd.h>
 
 namespace coreutils {
 
-   IMFBoundary(StreamReader &reader, ZString boundary) : boundary(boundary) {
-      
-   }
-      
-   int IMFBoundary::parse() {
-      switch(state) {
-       case PREFIX:
-	 if(buffer.remaining() > 3)          
-	   if(reader.nextIf("\r\n--")) 
-	     state = BOUNDARY;
-       case BOUNDARY:
-	 while(buffer.getTokenInclude(""));
-	 if(buffer.eod())
-	   return 2;
-	 else if(buffer.remaining() > 1)
-	   if(buffer.ifNext("--"))
-	     state = LAST_BOUNDARY;
-       case LAST_BOUNDARY:
-	 return 0;	 
+   IMFBoundary::IMFBoundary(StreamReader &reader, ZString boundary) : Parser(reader), boundary(boundary) {}
+
+   Parser::Status IMFBoundary::parse() {
+      MString &buffer = reader.getBuffer();
+      switch (state) {
+
+       case PREFIX: {
+          MString marker;
+          marker << "\r\n--";
+          marker << boundary;
+          int pos = buffer.find(marker);
+          if (pos < 0)
+             return Status::INCOMPLETE;
+          if (!buffer.ifNext(marker))
+             throw coreutils::Exception("IMFBoundary: boundary marker vanished between find and ifNext.");
+          state = TAIL;
+          return Status::DATA_AVAIL;
+       }
+
+       case TAIL:
+          // ZString::getLength() is already cursor-relative (bytes remaining
+          // from the cursor to the end) - do not subtract the cursor offset
+          // again here.
+          if (buffer.getLength() < 2)
+             return Status::INCOMPLETE;
+          if (buffer.ifNext(ZString("--"))) {
+             last = true;
+             return Status::COMPLETE;
+          }
+          if (buffer.ifNext(ZString("\r\n"))) {
+             last = false;
+             return Status::COMPLETE;
+          }
+          throw coreutils::Exception("IMFBoundary: malformed boundary line - expected CRLF or trailing --.");
+
       }
-      	
+      throw coreutils::Exception("IMFBoundary: unreachable state.");
    }
-            
-}
+
+   bool IMFBoundary::isLast() {
+      return last;
+   }
+
+}