Rebuild CGIFormData on Parser/StreamReader, fix a 2-byte-lookahead bug

CGIFormData now inherits Parser instead of MString and parses a CGI
request body (urlencoded or multipart/form-data) resumably against the
shared StreamReader, exposing the result via operator[] - matching every
other Parser in this redesign. multipart/form-data reuses IMFBoundary and
IMFHeader via checkParse() (chained through the same StreamReader) for
each part's boundary line and header block; urlencoded bodies wait for
the full Content-Length before delegating to the existing
CGIFormattedData parser. File-upload parts (Content-Disposition with a
filename= parameter) are recognized and skipped, matching this
codebase's convention that uploads get a separate mechanism.

Values are stored under an indexed "name:N" key (N = 0-based occurrence
count for that name), matching the :N-suffix convention JetCore's
Tag::getVariable already applies to its $[:name] CGI variable syntax
(defaulting to :0) - this also naturally supports repeated field names
(checkboxes, multi-select). operator[] applies the same :0 default so a
bare name still works when called directly, not just through that
convention layer.

Two real resumability bugs found and fixed via a byte-at-a-time test
(feeding one byte per StreamReader::feed() call, the harshest possible
fragmentation):
- CGIFormData's opening boundary marker ("--boundary", no leading CRLF
  per RFC 2046 for the first boundary) crammed "find+consume the marker"
  and "disambiguate -- vs CRLF" into one state; when the disambiguation
  needed more data, INCOMPLETE was returned without recording that the
  marker was already consumed, so retrying re-searched for it past where
  it had already been consumed, silently skipping content forever. Split
  into two states (INIT / OPENING_TAIL), the same fix already applied
  once to IMFBoundary's PREFIX/TAIL split.
- IMFHeader's FIELD state checked eod() (a 1-byte availability check)
  immediately before an ifNext("\r\n") call that needs 2 bytes - a lone
  trailing \r with no \n yet available was wrongly treated as malformed
  instead of "wait for more data". Fixed to check getLength() < 2.

Verified via a dedicated test (64 assertions total across this and the
earlier Parser/StreamReader test) covering urlencoded, multipart with a
skipped file upload, and the byte-at-a-time stress case - plus a real
end-to-end run through jet-2.0 with a simulated CGI multipart POST
request (see the matching JetCore commit).

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

IMFHeader.cpp

@@ -40,7 +40,11 @@ namespace coreutils {
 
        case FIELD:
           field << buffer.getTokenExclude("\r\n");
-          if (buffer.eod())
+          // Need 2 bytes buffered to reliably test for "\r\n" - eod() (a
+          // 1-byte check) would wrongly treat a lone trailing \r as
+          // malformed instead of "wait for the \n". Same bug class already
+          // fixed once in IMFBoundary's TAIL state.
+          if (buffer.getLength() < 2)
              return Status::INCOMPLETE;
           if (!buffer.ifNext(ZString("\r\n")))
              throw coreutils::Exception("IMFHeader: expected CRLF at end of header field.");