flushleft structure numbers

whatever.tex

\setuphead       [chapter,section] [alternative=leftalignmargin]
\setupmargindata [left]            [align=flushleft]
\setuplayout     [leftmargin=1cm]
\unprotect
%%% simplified from strc-ren.mkiv
\unexpanded\def\placeheadleftalignmarginalternative#1#2{%
  \vbox{%
    \localheadsetup\begstrut\dontleavehmode
      \ifconditional\headshownumber\inleftmargin{#1}\fi
      {#2}%
  }
}
\defineheadplacement[leftalignmargin] [\v!vertical]#1#2{%
  \placeheadleftalignmarginalternative{#1}{#2}%
}
\protect

\starttext \chapter{foo}\section{bar}\input knuth \stoptext