Add egui_inspection protocol and plugin (#8234)

Introduces live inspection for running egui apps over a small TCP
request/response protocol, plus the `egui::Plugin` that serves it.

This is the minimal surface to get the egui mcp in, we may want to
extend this in the future to add support for the inspection gui.

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/egui/src/context.rs

@@ -897,7 +897,7 @@ impl Context {
         profiling::function_scope!();
 
         let plugins = self.read(|ctx| ctx.plugins.ordered_plugins());
-        plugins.on_input(&mut new_input);
+        plugins.on_input(self, &mut new_input);
 
         self.write(|ctx| ctx.begin_pass(new_input));
     }
@@ -2391,7 +2391,7 @@ impl Context {
         let mut output = self.write(|ctx| ctx.end_pass());
 
         let plugins = self.read(|ctx| ctx.plugins.ordered_plugins());
-        plugins.on_output(&mut output);
+        plugins.on_output(self, &mut output);
 
         output
     }

crates/egui/src/plugin.rs

@@ -35,13 +35,13 @@ pub trait Plugin: Send + Sync + std::any::Any + 'static {
     ///
     /// Useful to inspect or modify the input.
     /// Since this is called outside a pass, don't show ui here. Using `Context::debug_painter` is fine though.
-    fn input_hook(&mut self, input: &mut RawInput) {}
+    fn input_hook(&mut self, ctx: &Context, input: &mut RawInput) {}
 
     /// Called just before the output is passed to the backend.
     ///
     /// Useful to inspect or modify the output.
     /// Since this is called outside a pass, don't show ui here. Using `Context::debug_painter` is fine though.
-    fn output_hook(&mut self, output: &mut FullOutput) {}
+    fn output_hook(&mut self, ctx: &Context, output: &mut FullOutput) {}
 
     /// Called when a widget is created and is under the pointer.
     ///
@@ -168,17 +168,17 @@ impl PluginsOrdered {
         });
     }
 
-    pub fn on_input(&self, input: &mut RawInput) {
+    pub fn on_input(&self, ctx: &Context, input: &mut RawInput) {
         profiling::scope!("plugins", "on_input");
         self.for_each_dyn(|plugin| {
-            plugin.input_hook(input);
+            plugin.input_hook(ctx, input);
         });
     }
 
-    pub fn on_output(&self, output: &mut FullOutput) {
+    pub fn on_output(&self, ctx: &Context, output: &mut FullOutput) {
         profiling::scope!("plugins", "on_output");
         self.for_each_dyn(|plugin| {
-            plugin.output_hook(output);
+            plugin.output_hook(ctx, output);
         });
     }