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/PluginHooks.hpp

@@ -0,0 +1,21 @@
+#pragma once
+
+// The plugin layer's installers for the hooks libslic3r exposes. libslic3r
+// stays free of any plugin/Python dependency: it exposes static setter seams
+// (ConfigBase::set_resolve_capability_fn, Print::set_slicing_pipeline_hook_fn,
+// ...) and this unit injects the dispatchers -- one file-local installer per
+// hook, aggregated by install(). Capabilities dispatched from the GUI layer
+// (e.g. PostProcessor.cpp) call execute_capabilities_from_refs at their own
+// call site and need no hook here.
+
+namespace Slic3r::plugin_hooks {
+
+// Install every hook. Called once from PluginManager::initialize().
+void install();
+
+// Reset every hook to null so none can enter Python after the interpreter
+// finalizes. Called from PluginManager::shutdown(); callers must have stopped
+// background slicing first (resetting a hook while process() runs is a race).
+void uninstall();
+
+} // namespace Slic3r::plugin_hooks