runtime: http_set_handler self-registers via dlsym

El programs that define `fn handle_request(method, path, body) -> String`
can now use http_serve directly without C-level glue. http_set_handler
falls back to dlsym(RTLD_DEFAULT, name) when the named handler isn't
already in the registry, picks up the El-compiled symbol, and registers
it transparently.

Closes the gap that made http_serve unusable from pure El. Verified
with a real El server on :17890 — POST /hello with body returned
{"method":"POST","path":"/hello","echo":"test body"} via curl.

dist/platform/elc rebuilt; .prev3 preserved.

el-compiler/runtime/el_runtime.c

@@ -27,6 +27,7 @@
 #include <sys/stat.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
+#include <dlfcn.h>      /* dlsym for http_set_handler fallback */
 #include <unistd.h>
 #include <dirent.h>
 #include <errno.h>
@@ -616,6 +617,24 @@ void http_set_handler(el_val_t name) {
     pthread_mutex_lock(&_http_handler_mu);
     free(_http_active_handler);
     _http_active_handler = el_strdup(n ? n : "");
+    /* If the name is not yet in the registry, try dlsym lookup against
+     * the running binary's symbol table. Every El `fn name(...)` compiles
+     * to a global C symbol with that exact name, so El programs can self-
+     * register their own handlers just by calling http_set_handler("name"). */
+    if (n && *n) {
+        int found = 0;
+        for (size_t i = 0; i < _http_handler_count; i++) {
+            if (strcmp(_http_handlers[i].name, n) == 0) { found = 1; break; }
+        }
+        if (!found) {
+            void* sym = dlsym(RTLD_DEFAULT, n);
+            if (sym && _http_handler_count < sizeof(_http_handlers) / sizeof(_http_handlers[0])) {
+                _http_handlers[_http_handler_count].name = el_strdup(n);
+                _http_handlers[_http_handler_count].fn   = (http_handler_fn)sym;
+                _http_handler_count++;
+            }
+        }
+    }
     pthread_mutex_unlock(&_http_handler_mu);
 }