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.
