refactor(plugin): move libslic3r hook wiring out of GUI_App into PluginHooks

GUI_App::on_init_inner() carried the plugin dispatch policy inline (the
capability resolver and the slicing-pipeline dispatcher) and would grow
with every capability that fires from inside libslic3r.

plugin/PluginHooks.{hpp,cpp} now owns one file-local installer per hook,
aggregated by plugin_hooks::install() -- called from
PluginManager::initialize(), reset in shutdown() so no hook can enter
Python after the interpreter finalizes. The wx-side loader subscriptions
move into GUI_App::init_plugin_gui_wiring().

No behavior change; dispatch bodies moved verbatim.

src/slic3r/plugin/PluginManager.cpp

@@ -5,6 +5,7 @@
 
 #include "PythonPluginBridge.hpp"
 #include "PluginFsUtils.hpp"
+#include "PluginHooks.hpp"
 #include "PythonFileUtils.hpp"
 
 #include <boost/log/trivial.hpp>
@@ -120,6 +121,10 @@ bool PluginManager::initialize()
 
     m_initialized = true;
 
+    // Install the libslic3r hooks (capability resolver, slicing-pipeline
+    // dispatcher). Uninstalled in shutdown() before the interpreter finalizes.
+    plugin_hooks::install();
+
     // Persist auto-load / capability state to each plugin's .install_state.json sidecar.
     // On load: write enabled=true plus current capability flags. On unload: flip enabled=false.
     // The on-unload callback is skipped during shutdown (run_on_unload_callbacks is gated by
@@ -154,6 +159,10 @@ void PluginManager::shutdown()
 
     BOOST_LOG_TRIVIAL(info) << __FUNCTION__ << ": PluginManager shutdown enter";
 
+    // Detach the libslic3r hooks first so nothing dispatches into Python while
+    // (or after) plugins unload. Callers stop background slicing before this.
+    plugin_hooks::uninstall();
+
     // Signal the loader to reject new plugin loads before we drain.
     m_loader.set_shutting_down();