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/compiler_lib.zig

@@ -58,6 +58,9 @@ pub const bound_fns = [_]BoundFn{
     .{ .sx_name = "declare_type", .handler = handleDeclareType },
     .{ .sx_name = "pointer_to", .handler = handlePointerTo },
     .{ .sx_name = "register_type", .handler = handleRegisterType },
+    // ── BuildOptions (migrated off `#compiler` onto `abi(.compiler)`) ─────────
+    .{ .sx_name = "build_options", .handler = handleBuildOptions },
+    .{ .sx_name = "set_post_link_callback", .handler = handleSetPostLinkCallback },
 };
 
 // Kind codes accepted by `register_type` — mirror `TypeTable.kindCode`. An
@@ -307,3 +310,27 @@ fn memberPair(elem: Value) ?struct { name: []const u8, ty: types.TypeId } {
     const ty = f[1].asTypeId() orelse return null;
     return .{ .name = name, .ty = ty };
 }
+
+// ── BuildOptions handlers (legacy dual-path, gate-OFF) ──────────────────────
+// The `abi(.compiler)` re-expression of `build_options` + `set_post_link_callback`,
+// reading the build config off the interpreter (`interp.build_config`). The VM
+// services the same names in `comptime_vm.callCompilerFn`; both stay in lockstep.
+
+/// `build_options() -> BuildOptions` — hand back the opaque zero-field handle. The
+/// state lives on `interp.build_config`; the handle is never dereferenced.
+fn handleBuildOptions(_: *Interpreter, _: []const Value) InterpError!Value {
+    return .void_val;
+}
+
+/// `set_post_link_callback(self, cb)` — record the callback `FuncId` on the build
+/// config so `main.zig` re-enters the evaluator post-link. The `cb` arg is a
+/// `.func_ref` value.
+fn handleSetPostLinkCallback(interp: *Interpreter, args: []const Value) InterpError!Value {
+    if (args.len != 2) return error.TypeError;
+    const bc = interp.build_config orelse return error.CannotEvalComptime;
+    switch (args[1]) {
+        .func_ref => |id| bc.post_link_callback_fn = id,
+        else => return error.TypeError,
+    }
+    return .void_val;
+}