P5.7 Step D: re-express metatype define() as sx over register_type

define(handle, info) is now an ordinary sx fn in modules/std/meta.sx: it matches
the TypeInfo union and calls the abi(.compiler) register_type primitive with the
matching kind code, decoding the variant/field/element list into []Member. An
all-void enum variant set registers as kind 2 (actual enum); any payload variant
as kind 3 (tagged_union).

To support matching the TypeInfo VALUE in the comptime VM, added tagged-union
value support: kindOf now treats tagged_union as a by-address aggregate, enum_tag
reads the tag word at offset 0, and a new enum_payload arm reads the active
payload at tag_size (both bail loudly on backing_type unions, whose layout
differs). register_type's duplicate-name diagnostics now include the offending
name. Dropped the define interception in tryLowerReflectionCall; the .enum(...)
arg infers TypeInfo from the sx fn's param type via the ordinary call path.

Regenerated 1179/1180 diagnostic snapshots (same span/line; the message now
names register_type instead of define()). define/type_info builtins still exist
pending dead-code removal.

examples/expected/1179-diagnostics-comptime-type-construction-bail.stderr

@@ -1,4 +1,4 @@
-error: comptime type construction failed: comptime define(): enum has no variants
+error: comptime type construction failed: comptime register_type: a type with no members is never valid
   --> examples/1179-diagnostics-comptime-type-construction-bail.sx:14:10
    |
 14 | Empty :: define(declare("Empty"), .enum(.{ variants = .[] }));

examples/expected/1180-diagnostics-metatype-duplicate-variant.stderr

@@ -1,4 +1,4 @@
-error: comptime type construction failed: comptime define(): duplicate variant name 'value'
+error: comptime type construction failed: comptime register_type: duplicate member name 'value'
   --> examples/1180-diagnostics-metatype-duplicate-variant.sx:8:8
    |
  8 | Bad :: define(declare("Bad"), .enum(.{ variants = .[