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/1655-platform-asm-callback-into-sx.sx

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+// ASM stream — the round trip: sx → asm → sx. A global-asm routine (`_caller`)
+// CALLS BACK into an sx function (`cb`) by its symbol, then returns. For the asm
+// `bl _cb` to resolve, the sx callback needs EXTERNAL LINKAGE and a stable C
+// symbol — that is exactly what `export` provides (it also implies the C ABI, so
+// no hidden context parameter). `callconv(.c)` alone is NOT enough: it sets the
+// ABI but leaves the function `internal`, so it is dead-code-eliminated (nothing
+// in the IR references it — the `bl` is opaque to the optimizer) and `_cb` is
+// undefined at link. macOS gives `export "cb"` the symbol `_cb` (leading
+// underscore), which the template references. aarch64-macos-pinned; runs under
+// the JIT here (sx run), ir-only elsewhere.
+
+// The sx callback — `export` gives it external linkage + the `_cb` symbol + C ABI.
+cb :: (n: i64) -> i64 export "cb" {
+    return n + 1;
+}
+
+// A global-asm trampoline that calls back into `cb`. It saves/restores the link
+// register (x30) around the `bl` — it was itself reached via `bl`, so the return
+// address must survive the nested call.
+asm {
+    #string ASM
+.global _caller
+_caller:
+    stp x29, x30, [sp, #-16]!
+    bl _cb                      // x0 = cb(x0)  — back into sx
+    ldp x29, x30, [sp], #16
+    ret
+ASM,
+};
+
+caller :: (n: i64) -> i64 extern;
+
+main :: () -> i64 {
+    return caller(41);          // sx main → asm caller → bl _cb → sx cb → 42
+}

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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+