P5.7 Step D: re-express metatype declare() as sx over declare_type

declare(name) is now an ordinary sx fn in modules/std/meta.sx that calls the
abi(.compiler) declare_type primitive — both mint/find the same forward nominal
slot. Removed the bespoke .declare arm from callBuiltinVm and the BuiltinId.declare
member; dropped the declare interception in tryLowerReflectionCall (the call now
routes to the sx fn). preregisterForwardTypes still scans for the literal
declare("Name") spelling so *Name self-references forward-register before the
body lowers (0618). define/type_info/field_type remain builtins.

src/ir/inst.zig

@@ -452,17 +452,14 @@ pub const BuiltinId = enum(u16) {
     type_eq,
     type_is_unsigned,
     has_impl,
-    // The compiler's ONLY comptime type-CONSTRUCTION primitives. Higher-level
+    // The comptime type-CONSTRUCTION terminator builtin. Higher-level
     // constructors (one-shot, channel-result, etc.) are ordinary sx built over
-    // these — the compiler knows none of them by name. Both are comptime-only
-    // (the interp mutates the type table via its
-    // `mint` handle); reaching them at runtime / emit is a hard error.
-    //   declare()            → mint an EMPTY (undefined) nominal slot, returned
-    //                          as a `Type` value. Using the slot before its
-    //                          `define` is a loud diagnostic.
+    // it — the compiler knows none of them by name. Comptime-only (the comptime
+    // VM mutates the type table); reaching it at runtime / emit is a hard error.
+    // (`declare` is no longer a builtin — it's plain sx over the `declare_type`
+    // compiler-API primitive in `modules/std/meta.sx`.)
     //   define(handle, info) → decode the `TypeInfo` VALUE (the name travels in
     //                          it) and complete the slot.
-    declare,
     define,
     // The comptime reflection INVERSE of `define`: read a type's variants
     // (name + payload type) out of the type table and CONSTRUCT the same