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/sema.zig

@@ -1001,7 +1001,7 @@ pub const Analyzer = struct {
         }
 
         // Built-in names that aren't declared in source
-        const builtins = [_][]const u8{ "io", "true", "false", "cast", "closure", "out", "size_of", "align_of", "malloc", "free", "memcpy", "memset", "context" };
+        const builtins = [_][]const u8{ "io", "true", "false", "cast", "closure", "size_of", "align_of", "malloc", "free", "memcpy", "memset", "context" };
         for (builtins) |b| {
             if (std.mem.eql(u8, name, b)) return;
         }