M7 fixtures
M7 adds optionals and error unions on top of the M6 ownership model.
ok*.fe must compile. bad*.fe must fail according to the first-line error marker.
The files are not wired into TEST-DOS.BAT until M7 work begins.
Coverage: contextual null, ?T, .?, Some/None pattern-only non-destructive views, mem.replace extraction, lazy orelse/catch, nominal error unions, try, block/short catch, and error code/name uniqueness, plus R4/R7 interactions with optional references/owners.