Adds examples/1655-platform-asm-callback-into-sx.sx: a global-asm
trampoline (_caller) that `bl _cb` back into an `export`ed sx function.
Demonstrates the sx → asm → sx round trip and that `export` (external
linkage + stable C symbol + C ABI) is what makes the callback symbol
resolvable — `callconv(.c)` alone leaves it internal and it DCE's away.
Runs under the JIT on aarch64-macos (→ 42); ir-only elsewhere. Locks
current behavior; no compiler change.
