test(asm): reject symbol "s" operands cleanly + lock (symbol-op prep)

A symbol operand (constraint "s") feeds a function/global symbol whose
mangled name the template emits — enabling a DIRECT `bl %[fn]` (one
fewer indirection than register-indirect `blr`). Until now `"s" = fn`
fell through to emit and produced an LLVM-verifier crash (param type
mismatch). Reject it at lowering with a clear diagnostic instead, and
lock that with examples/1656-platform-asm-symbol-operand.sx. The next
commit implements it and flips the example to run (→ 42).

examples/expected/1656-platform-asm-symbol-operand.stdout

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