export fn and finch build --shared: libraries a C program loads (game scripts)

export fn name(...) is a C function by that name (numbers, bool, char, pointers in and out).
--shared builds lib<name>.so / <name>.dll with no main. finch_module_on_fatal(handler) lets the
host take runtime errors over (longjmp out) instead of the process exiting.
tests/shared.sh: a C host loads one, calls it and survives an index error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/shared/host.c

@@ -0,0 +1,17 @@
+#include <dlfcn.h>
+#include <setjmp.h>
+#include <stdio.h>
+static jmp_buf jb;
+void host_print(const char *s) { printf("host_print: %s\n", s); }
+static void onFatal(const char *m) { printf("caught: %s\n", m); longjmp(jb, 1); }
+int main(int argc, char **argv) {
+    void *h = dlopen(argc > 1 ? argv[1] : "./libmod.so", RTLD_NOW | RTLD_LOCAL);
+    if (!h) { printf("%s\n", dlerror()); return 1; }
+    ((void (*)(void (*)(const char *)))dlsym(h, "finch_module_on_fatal"))(onFatal);
+    printf("add %ld\n", ((long (*)(long, long))dlsym(h, "add"))(2, 40));
+    printf("even %d %d\n", ((_Bool (*)(long))dlsym(h, "is_even"))(4), ((_Bool (*)(long))dlsym(h, "is_even"))(5));
+    ((void (*)(const char *))dlsym(h, "hello"))("Rakya");
+    if (!setjmp(jb)) ((long (*)(long))dlsym(h, "crash"))(7);
+    printf("still alive\n");
+    return 0;
+}