feat(parser): reserved keyword as member name after .

After a leading `.` (enum literal `.enum`, field access `x.enum` /
`E.struct`, match arm `case .enum:`) a reserved keyword is unambiguously
the member/variant NAME — the dot rules out the keyword reading — so no
backtick escape is needed. A declaration of such a variant still needs
the backtick (enum { `enum: i64 }), since the decl site has no dot.

Adds Parser.dotMemberName() (identifier OR identifier-shaped keyword)
and routes the leading-dot enum-literal and postfix field-access sites
through it. readme updated. The reify example 0614 now uses the cleaner
reify(.enum(...)) spelling (still xfail — reify lands next commit).

examples/0614-comptime-reify-enum.sx

@@ -8,7 +8,7 @@
 #import "modules/std.sx";
 #import "modules/std/meta.sx";
 
-E :: reify(.`enum(.{ variants = .[
+E :: reify(.enum(.{ variants = .[
     EnumVariant.{ name = "value",  payload = i64  },
     EnumVariant.{ name = "closed", payload = void },
 ] }));