Add egui_inspection crate and eframe inspection hooks

New `egui_inspection` crate ships:
- `protocol` (default): wire types + length-prefixed msgpack framing for the
  inspector ↔ egui-peer connection. Transport-neutral (stdio / unix socket / TCP).
- `plugin`: `InspectionPlugin`, an `egui::Plugin` that dials a unix socket from
  `EGUI_INSPECTION_SOCKET`, streams frames + accesskit tree updates, and applies
  inbound `InspectorCommand`s back into the running `egui::Context`.

eframe gains an `inspection` feature that auto-attaches the plugin during native
startup (glow + wgpu integrations) when the env var is set. Connection failures
log via `log::warn!` and do not abort startup.

Lives in its own crate (rather than `egui_kittest`) so eframe can pull the
protocol in without picking up the test harness, and so external tools can
depend on it directly.

crates/egui_inspection/README.md

@@ -0,0 +1,15 @@
+# egui_inspection
+
+Wire protocol and `egui::Plugin` for live inspection of running egui apps and
+kittest harnesses.
+
+Two layers:
+
+- **`protocol`** (default feature): length-prefixed MessagePack messages used by
+  `egui_kittest`'s inspector, the external `kittest_inspector` UI, and the
+  `egui_kittest_mcp` server.
+
+- **`plugin`** (opt-in): an `egui::Plugin` implementation that streams frames +
+  AccessKit tree updates to an inspector over a unix domain socket and applies
+  received `InspectorCommand`s back into the running app. Auto-attaches when
+  `EGUI_INSPECTION_SOCKET` is set.