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>

IMFFormData.cpp

@@ -3,9 +3,15 @@
 #include "IMFText.h"
 
 namespace coreutils {
-   
+
    IMFFormData::IMFFormData(int fd, ZString boundary) {
-      IMFHeader header(fd);
+      // TODO: IMFHeader now reads through a StreamReader (see the Parser/
+      // StreamReader redesign) instead of a raw fd. This constructor - and
+      // IMFMultipart, which still constructs IMFFormData from a bare fd -
+      // needs migrating to StreamReader together before header parsing can
+      // be wired back in here. Out of scope for that redesign pass; this
+      // class was already an unfinished stub (constructed objects were
+      // discarded, nothing was stored) before this change.
       IMFText text(fd, boundary);
    }