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.exit

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+1

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

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+error: symbol asm operands (`"s"`) are not yet implemented
+  --> examples/1656-platform-asm-symbol-operand.sx:10:12
+   |
+10 |     return asm volatile {
+   |            ^^^^^^^^^^^^^^
+11 |         #string ASM
+   | ^^^^^^^^^^^^^^^^^^^
+12 |         stp x29, x30, [sp, #-16]!
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+13 |         mov x0, %[arg]
+   | ^^^^^^^^^^^^^^^^^^^^^^
+14 |         bl %[fn]
+   | ^^^^^^^^^^^^^^^^
+15 |         mov %[res], x0
+   | ^^^^^^^^^^^^^^^^^^^^^^
+16 |         ldp x29, x30, [sp], #16
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+17 | ASM,
+   | ^^^^
+18 |         [res] "=r" -> i64,
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+19 |         [arg] "r" = n,
+   | ^^^^^^^^^^^^^^^^^^^^^^
+20 |         [fn]  "s" = cb,             // symbol operand → direct `bl _cb`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+21 |         clobbers(.x0, .x30, .memory),
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+22 |     };
+   | ^^^^^

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

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+