feat(metatype): make_enum — general enum constructor over a []EnumVariant value

make_enum(name, variants: []EnumVariant) -> Type mints a nominal enum
from a variant list passed as a VALUE, not a hardcoded literal — the
open-ended form the channel-result constructors are special cases of.
Pure sx over declare/define; no compiler machinery.

Because variants is an ordinary comptime value, a non-generic builder
can ASSEMBLE it in a local before minting. examples/0620: build_level
fills a local array, then make_enum mints Level from it — exercising
define decoding a value-arg SLICE (decodeVariantElements' slice branch),
vs. the inline .[ … ] array the 0614-0618 examples pass directly.

No compiler change (locks existing capability). Suite green (678).

examples/expected/0620-comptime-metatype-make-enum.exit

@@ -0,0 +1 @@
+0

examples/expected/0620-comptime-metatype-make-enum.stderr

@@ -0,0 +1 @@
+

examples/expected/0620-comptime-metatype-make-enum.stdout

@@ -0,0 +1,3 @@
+info
+warn
+fatal code=70