test(asm): round-trip example — asm calls back into an sx function

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.

examples/expected/1655-platform-asm-callback-into-sx.build

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+{ "target": "macos" }

examples/expected/1655-platform-asm-callback-into-sx.exit

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+42

examples/expected/1655-platform-asm-callback-into-sx.ir

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+
+module asm ".global _caller"
+module asm "_caller:"
+module asm "    stp x29, x30, [sp, #-16]!"
+module asm "    bl _cb                      // x0 = cb(x0)  \E2\80\94 back into sx"
+module asm "    ldp x29, x30, [sp], #16"
+module asm "    ret"
+
+; Function Attrs: nounwind
+define i64 @cb(i64 %0) #0 {
+entry:
+  %alloca = alloca i64, align 8
+  store i64 %0, ptr %alloca, align 8
+  %load = load i64, ptr %alloca, align 8
+  %add = add i64 %load, 1
+  ret i64 %add
+}
+
+; Function Attrs: nounwind
+declare i64 @caller(i64) #0
+
+; Function Attrs: nounwind
+define i32 @main() #0 {
+entry:
+  %call = call i64 @caller(i64 41)
+  %ca.tr = trunc i64 %call to i32
+  ret i32 %ca.tr
+}

examples/expected/1655-platform-asm-callback-into-sx.stderr

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+

examples/expected/1655-platform-asm-callback-into-sx.stdout

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+