comptime VM arc: abi(.compiler) ABI, out as sx fn, VM-native diagnostics, BuildConfig threaded

Lands the full VM/compiler-API arc on branch reify (701/0 both gates):
- abi(.compiler) ABI replaces abi(.zig) extern compiler + the fake
  #library "compiler"; bodiless decl = compiler-API surface, bodied =
  user compiler-domain fn (lowered for VM eval, emit-skipped).
- out is a plain sx fn (libc write) — the out builtin deleted; the VM
  handles it via host-FFI. trace_resolve + interp_print_frames ported.
- 4B VM-native diagnostics: 1179/1180 render proper comptime type
  construction failed: under strict.
- S5a: build_options/set_post_link_callback on abi(.compiler) with
  BuildConfig threaded into the VM (green intermediate).
- 0522 fixed (describe(args: []Type)); regression 0638.

Strict deletion-gate down to 4 compiler_call bails (1609/1614/1615/1616)
+ 1654 (legitimate unresolvable-symbol diagnostic).

src/ir/interp.zig

@@ -1940,13 +1940,6 @@ pub const Interpreter = struct {
 
     fn execBuiltinInner(self: *Interpreter, bi: inst_mod.BuiltinCall, frame: *Frame) InterpError!ExecResult {
         switch (bi.builtin) {
-            .out => {
-                const str_val = frame.getRef(bi.args[0]);
-                if (str_val.asString(self)) |s| {
-                    self.output.appendSlice(self.alloc, s) catch {};
-                }
-                return .{ .value = .void_val };
-            },
             .size_of => {
                 // Return a default size (8 bytes for most types)
                 return .{ .value = .{ .int = 8 } };