test(asm): pin loud failure of #run into a module-asm symbol

Adds examples/1654-platform-asm-global-comptime-call.sx — the comptime
guard. A module-asm symbol only exists after assemble+link; the comptime
interpreter resolves extern calls via host dlsym, where it's absent, so
`#run my_add(…)` fails with a clear diagnostic ("comptime extern call:
symbol not found via dlsym") rather than misfiring. Runtime calls work
(1648/1653). dlsym-miss precedes asm assembly, so arch-independent — no
.build. Locks current behavior; no compiler change.

examples/1654-platform-asm-global-comptime-call.sx

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+// ASM stream — calling a global-asm symbol at COMPILE TIME (`#run`) fails loud.
+// A module-asm symbol only exists once the module is assembled+linked; the
+// comptime interpreter resolves `extern` calls via host `dlsym` (RTLD_DEFAULT),
+// where the symbol is absent — so `#run my_add(…)` cannot evaluate and reports a
+// clear diagnostic instead of silently misfiring. (Calling the same symbol at
+// RUNTIME works under both JIT and AOT — see 1648/1653.) The failure is at
+// dlsym resolution, before any asm is assembled, so it is arch-independent —
+// no `.build` target needed. Regression guard for the comptime boundary.
+asm {
+    #string ASM
+.global _my_add
+_my_add:
+    add x0, x0, x1
+    ret
+ASM,
+};
+
+my_add :: (a: i64, b: i64) -> i64 extern;
+
+COMPUTED :: #run my_add(40, 2);     // compile-time call — module-asm symbol not yet linked
+
+main :: () -> i64 { return COMPUTED; }

examples/expected/1654-platform-asm-global-comptime-call.exit

@@ -0,0 +1 @@
+1

examples/expected/1654-platform-asm-global-comptime-call.stderr

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+error: comptime init of 'COMPUTED' failed: CannotEvalComptime (op=call: comptime extern call: symbol not found via dlsym (target-specific binding called at compile time?))

examples/expected/1654-platform-asm-global-comptime-call.stdout

@@ -0,0 +1 @@
+