ffi 0.0: tests/cross_compile.sh scaffold

First step of the FFI ceremony reduction plan (current/PLAN-FFI.md).
Iterates a (target, example) tuple list, runs `sx build --target <t>
<example>`, asserts exit 0 + output file produced. Cross-compile
correctness only — these examples can't run on the host.

Initial tuple list is empty, so the script exits 0 on a clean tree
and contributors without the iOS SDK / Android NDK aren't blocked.
`toolchain_available` short-circuits with a SKIP line when the
requested toolchain isn't installed. Phase 1/2/3 cross-only examples
populate TUPLES as they land.

tests/cross_compile.sh

@@ -0,0 +1,77 @@
+#!/bin/bash
+# Cross-compile regression runner.
+#
+# For each (target, example) tuple, runs `./sx build --target <t> <example>`
+# and asserts (a) exit 0 and (b) the expected output file was produced.
+# Compile correctness only — these examples can't be executed on the host
+# (iOS Obj-C runtime / Android NDK).
+#
+# Tuple list starts empty and grows as Phase 0 / 1 / 2 / 3 of the FFI plan
+# add cross-only examples. Skips with a warning (still exits 0) when the
+# required toolchain isn't installed, so contributors without the iOS SDK
+# or Android NDK aren't blocked.
+#
+# Usage: ./tests/cross_compile.sh
+
+set -uo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+ROOT_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
+SX="$ROOT_DIR/zig-out/bin/sx"
+TMP_DIR="${TMPDIR:-/tmp}/sx-cross-compile"
+mkdir -p "$TMP_DIR"
+
+# Tuple format: "<target>|<example_path>"
+# Add entries as cross-only examples land. Empty for now — Phase 0 only
+# lays down the runner; baselines live in tests/run_examples.sh territory.
+TUPLES=()
+
+PASS=0
+FAIL=0
+SKIP=0
+
+toolchain_available() {
+    local target="$1"
+    case "$target" in
+        ios|ios-sim)
+            xcrun --sdk iphonesimulator --show-sdk-path >/dev/null 2>&1
+            ;;
+        android|android-arm64)
+            # discoverAndroidNdk in target.zig accepts $ANDROID_NDK_HOME,
+            # $ANDROID_NDK_ROOT, or a scan of $HOME/Library/Android/sdk/ndk.
+            [[ -n "${ANDROID_NDK_HOME:-}" || -n "${ANDROID_NDK_ROOT:-}" ]] \
+                || [[ -d "$HOME/Library/Android/sdk/ndk" ]]
+            ;;
+        *)
+            return 1
+            ;;
+    esac
+}
+
+for tuple in "${TUPLES[@]:-}"; do
+    [[ -z "$tuple" ]] && continue
+    target="${tuple%%|*}"
+    example="${tuple#*|}"
+    label="$target / $(basename "$example" .sx)"
+
+    if ! toolchain_available "$target"; then
+        SKIP=$((SKIP + 1))
+        printf "  %-50s SKIP (no toolchain)\n" "$label"
+        continue
+    fi
+
+    out_obj="$TMP_DIR/$(basename "$example" .sx).$target.o"
+    printf "  %-50s" "$label"
+    "$SX" build --target "$target" -o "$out_obj" "$ROOT_DIR/$example" >/dev/null 2>&1
+    rc=$?
+    if [[ $rc -eq 0 && -s "$out_obj" ]]; then
+        PASS=$((PASS + 1))
+        echo "ok"
+    else
+        FAIL=$((FAIL + 1))
+        echo "FAIL (exit=$rc, output=$out_obj)"
+    fi
+done
+
+echo "$PASS passed, $FAIL failed, $SKIP skipped"
+[[ $FAIL -eq 0 ]]