fix: diagnose missing 'main' instead of segfaulting on 'sx run' (issue 0137)

A program with no 'main' reached the JIT entry-point call with a garbage
address (ORC reports lookup success but leaves main_addr degenerate), then
called it -> SIGSEGV. Add a pre-JIT entry-point check in main.zig that emits
'error: no main function found' and exits non-zero before codegen, plus a
defensive main_addr==0 guard in target.zig runJITFromObject as a backstop.

Regression: examples/1188-diagnostics-run-no-main.sx

src/target.zig

@@ -383,6 +383,17 @@ pub fn runJITFromObject(obj_buf: c.LLVMMemoryBufferRef, priority_dylibs: []const
         return error.CompileError;
     }
 
+    // Defensive backstop (issue 0137): ORC has been observed reporting
+    // success from the `main` lookup while leaving `main_addr` at 0 — calling
+    // @ptrFromInt(0) then segfaults. The real fix is the pre-JIT entry-point
+    // check in main.zig (which also catches the observed NON-zero garbage
+    // address case); this guard is a last line of defense so a null entry can
+    // never be called regardless of how we got here.
+    if (main_addr == 0) {
+        std.debug.print("error: no 'main' function found in JIT module\n", .{});
+        return error.CompileError;
+    }
+
     // Cast to function pointer and call. The exit code is main's integer
     // return truncated to u8 — matching the OS truncation an AOT binary's
     // exit status already gets, so JIT and AOT agree (e.g. 1105 -> 81,