Modernize C builds with offline dependencies and portable runtime helpers (#3050)

Bundle pinned offline dependencies, remove libcork/libipset and submodule requirements, and add portable runtime helpers with relocatable static/shared library installations.

Add Clang static build validation for Linux/musl, macOS and Windows, real TCP/UDP and SIP003 interoperability coverage, and recorded performance tradeoffs. All 21 hosted checks pass at the reviewed PR head 8d504e6218.

src/plugin_disabled.c

@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+#include <stdint.h>
+#include "plugin.h"
+#include "utils.h"
+int
+start_plugin(const char *plugin, const char *options, const char *remote_host,
+             const char *remote_port, const char *local_host, const char *local_port,
+#ifdef __MINGW32__
+             uint16_t control_port,
+#endif
+             enum plugin_mode mode)
+{
+    (void)plugin; (void)options; (void)remote_host; (void)remote_port;
+    (void)local_host; (void)local_port; (void)mode;
+#ifdef __MINGW32__
+    (void)control_port;
+#endif
+    LOGE("Plugin support is disabled in this build");
+    return -1;
+}
+uint16_t get_local_port(void) { FATAL("Plugin support is disabled in this build"); return 0; }
+void stop_plugin(void) {}
+int is_plugin_running(void) { return 0; }