test(ffi-linkage): xfail export fn called from C via AOT (Phase 2.0)

Phase 2 of the extern/export stream verifies `export` (define + expose a
C-ABI sx symbol) end-to-end. C->sx-by-name linkage cannot work under the
corpus's `sx run` JIT mode — a JIT-resident symbol is invisible to a
dlopen'd C dylib's flat-namespace lookup — so this lands a new AOT
execution mode for the corpus: an `expected/<name>.aot` marker switches an
example from JIT `sx run` to a `sx build` + execute flow, linking the sx
object with its C `#source` companions into a native binary.

example/1226 defines `sx_square :: (n: i32) -> i32 export { ... }` and a
companion .c that declares `extern int sx_square(int)` and calls it back.
RED: with `export` not yet lowered, the AOT link fails with an undefined
`_sx_square` (the define path still emits it `internal` + with an implicit
ctx slot, and lazy lowering leaves an uncalled export fn as a bodiless
declare). Phase 2.1 greens it.

Also retires the standalone `tests/run_examples.sh` runner — `zig build
test` (src/corpus_run.test.zig) is now the sole corpus runner, and the
shell mirror would have needed its own AOT-mode port to stay in lockstep.
verify-step.sh drops its redundant step (zig build test already runs the
corpus); CLAUDE.md documents the `.aot` mode.

examples/1226-ffi-export-fn.c

@@ -0,0 +1,8 @@
+#include "1226-ffi-export-fn.h"
+
+// Defined on the sx side via `export` — a plain C-ABI symbol, no sx context.
+extern int sx_square(int n);
+
+int call_sx_square(int n) {
+    return sx_square(n) + 1;
+}

examples/1226-ffi-export-fn.h

@@ -0,0 +1,7 @@
+#ifndef SX_EXPORT_FN_H
+#define SX_EXPORT_FN_H
+
+// Calls back into the sx-exported `sx_square` and adds 1.
+int call_sx_square(int n);
+
+#endif

examples/1226-ffi-export-fn.sx

@@ -0,0 +1,28 @@
+// export function (FFI-linkage stream, Phase 2): define an sx function with
+// the bare `export` linkage modifier — external linkage + C ABI + no sx ctx —
+// so a companion C translation unit can call back into it by its plain symbol
+// name. The C side (`#source`) declares `sx_square` as a normal `extern int`
+// and calls it; sx `main` drives the C side via `call_sx_square`. Mirrors the
+// import-direction `extern` examples (1223–1225) for the define direction.
+//
+// Without `export`, an sx-defined fn is `internal` linkage + carries the
+// implicit `__sx_ctx` slot, so the C object can neither resolve nor correctly
+// call the symbol — this is the gap `export` fills.
+#import "modules/std.sx";
+
+#import c {
+    #include "1226-ffi-export-fn.h";
+    #source "1226-ffi-export-fn.c";
+};
+
+// sx-defined, exported to C: external linkage + C ABI + no implicit ctx.
+sx_square :: (n: i32) -> i32 export {
+    return n * n;
+}
+
+main :: () -> i32 {
+    // call_sx_square (C) calls back into sx_square, adds 1.
+    print("call_sx_square(6) = {}\n", call_sx_square(6));
+    print("call_sx_square(9) = {}\n", call_sx_square(9));
+    0
+}

examples/expected/1226-ffi-export-fn.exit

@@ -0,0 +1 @@
+0

examples/expected/1226-ffi-export-fn.stdout

@@ -0,0 +1,2 @@
+call_sx_square(6) = 37
+call_sx_square(9) = 82