lock(reify): meta.sx surface + bodyless #builtin decls + loud bails

REIFY Phase 0.0. Add the comptime type-metaprogramming surface as the
on-demand module modules/std/meta.sx (NOT the prelude — declaring its
data types in always-loaded core.sx interns them into every module's
type table and shifts every .ir snapshot):

  - EnumVariant / EnumInfo / TypeInfo data types. TypeInfo's variant uses
    the backtick raw escape `enum so it reads as the keyword.
  - reify / type_info / field_type as bodyless #builtin decls.

Each builtin bails LOUDLY when reached unimplemented (no silent default):
  - reify(...) in a :: type-alias position -> decl.zig .call branch
    (also the Phase 0.2 construction hook); poisons the alias .unresolved.
  - reify / field_type in any other type position ->
    generic.zig resolveTypeCallWithBindings.
  - type_info(...) in expression position -> call.zig tryLowerReflectionCall.

Unit test src/parser.test.zig (registered in root.zig) locks that the
decls parse. zig build test green (447 unit, 669 examples).

library/modules/std/core.sx

@@ -31,6 +31,11 @@ field_value_int :: ($T: Type, idx: i64) -> i64 #builtin;
 field_index :: ($T: Type, val: T) -> i64 #builtin;
 error_tag_name :: (e: $T) -> string #builtin;
 
+// Comptime type metaprogramming (`type_info` / `reify` / `field_type`) lives in
+// the on-demand `modules/std/meta.sx`, NOT here — declaring its data types in
+// the always-loaded prelude would intern them into every module's type table
+// and shift every `.ir` snapshot. Import `modules/std/meta.sx` to use reify.
+
 // Call-site location, synthesized by the `#caller_location` directive when it
 // is a parameter's default value (ERR E4.1b). `process.exit` / `assert` use it
 // to report where they were invoked.