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/ast.zig

@@ -132,9 +132,15 @@ pub const Root = struct {
 /// - `.zig` — welded to the real internal Zig type/fn: layout follows the bound
 ///   Zig type, functions dispatch over the comptime host-call bridge. The
 ///   `compiler` library (`design/comptime-compiler-api.md`) binds via `abi(.zig)`.
+/// - `.compiler` — a COMPILER-DOMAIN function: it runs in the comptime evaluator
+///   (VM / interp), NEVER in the shipped binary, so the backend does not lower it.
+///   Covers the compiler-API surface (`intern`/`find_type`/`build_options`/… —
+///   bodiless decls whose Zig/VM handler is the impl) AND user compiler-domain
+///   functions like post-link callbacks (bodied, but emit-skipped). The ABI alone
+///   marks it — there is no `extern <lib>` and no fake `#library "compiler"`.
 /// - `.pure` — a pure / naked function (inline asm body), no calling-convention
 ///   prologue/epilogue.
-pub const ABI = enum { default, c, zig, pure };
+pub const ABI = enum { default, c, compiler, pure };
 
 /// Linkage modifier written in the postfix slot before `abi(...)`:
 /// `name :: (sig) -> Ret [extern | export] [abi(.x)] [lib] [;|{…}];`