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).

library/modules/std/core.sx

@@ -4,7 +4,14 @@
 // never import this file directly — std.sx re-exports every name here.
 
 Vector :: ($N: int, $T: Type) -> Type #builtin;
-out :: (str: string) -> void #builtin;
+// `out` writes a string straight to fd 1 via libc `write` — a plain sx function,
+// NOT a compiler builtin. At comptime it runs through the evaluator's host-FFI
+// escape (the VM's dlsym path / the interp's extern call); at runtime it's an
+// ordinary libc call. `libc_write` is the raw escape hatch (cf. libc_malloc/free).
+libc_write :: (fd: i32, buf: [*]u8, count: usize) -> isize extern libc "write";
+out :: (str: string) -> void {
+    libc_write(1, str.ptr, xx str.len);
+}
 // sqrt :: (x: $T) -> T #builtin;
 // sin :: (x: $T) -> T #builtin;
 // cos :: (x: $T) -> T #builtin;