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).

src/root.zig

@@ -4,6 +4,7 @@ pub const lexer = @import("lexer.zig");
 pub const lexer_tests = @import("lexer.test.zig");
 pub const ast = @import("ast.zig");
 pub const parser = @import("parser.zig");
+pub const parser_tests = @import("parser.test.zig");
 pub const print = @import("print.zig");
 pub const types = @import("types.zig");
 pub const target = @import("target.zig");