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/build.sx

@@ -22,12 +22,6 @@ BuildOptions :: struct #compiler {
     asset_dir_src_at :: (self: BuildOptions, i: i64) -> string;
     asset_dir_dest_at :: (self: BuildOptions, i: i64) -> string;
 
-    // Post-link callback. Registers a sx function the compiler will
-    // invoke after `target.link()` returns. Used by the sx-side
-    // bundler (`platform.bundle.bundle_main`) and by user programs
-    // that want custom post-build steps. Return `false` to fail the build.
-    set_post_link_callback :: (self: BuildOptions, cb: () -> bool);
-
     // Name-based alternative to `set_post_link_callback`. The
     // compiler resolves `<module_name>.bundle_main` after linking.
     set_post_link_module :: (self: BuildOptions, module_name: [:0]u8);
@@ -91,4 +85,12 @@ BuildOptions :: struct #compiler {
     jni_main_java_source_at :: (self: BuildOptions, i: i64) -> string;
 }
 
-build_options :: () -> BuildOptions #compiler;
+build_options :: () -> BuildOptions abi(.compiler);
+
+// Post-link callback. Registers a sx function the compiler will invoke after
+// `target.link()` returns. Used by the sx-side bundler
+// (`platform.bundle.bundle_main`) and by user programs that want custom
+// post-build steps. Return `false` to fail the build. Migrated off `#compiler`
+// onto the comptime compiler-API (`abi(.compiler)`); called as
+// `opts.set_post_link_callback(cb)` via UFCS, from inside a `#run { … }` block.
+set_post_link_callback :: ufcs (self: BuildOptions, cb: () -> bool) abi(.compiler);

library/modules/platform/bundle.sx

@@ -24,7 +24,12 @@
 //     }
 // =====================================================================
 
-bundle_main :: () -> bool {
+// `bundle_main` is the post-link callback — it runs in the comptime evaluator
+// (interp/VM) after `target.link()`, NEVER in the shipped binary. Marked
+// `abi(.compiler)` so the backend doesn't lower it and its `build_options()`
+// compiler-API call is permitted (it would otherwise be rejected as a
+// comptime-only function called at runtime).
+bundle_main :: () -> bool abi(.compiler) {
     opts := build_options();
     binary := opts.binary_path();
     bundle := opts.bundle_path();

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;