egui_inspection: PNG-encode screenshots on the wire; collapse protocol feature

- FrameScreenshot now carries PNG bytes instead of raw RGBA (PROTOCOL_VERSION 1→2);
  add a shared `encode_png` helper behind a new `png` feature so the live plugin and the
  kittest harness encode frames identically.
- Make the protocol module unconditional: drop the `protocol` feature flag and the
  optional serde/serde_bytes/rmp-serde deps it gated.
- plugin.rs: re-stamp screenshot-bearing frames with the current step (so inspectors
  waiting for step > prev don't reject them) and pump a tail-side repaint while awaiting
  the GPU readback.

crates/egui_inspection/Cargo.toml

@@ -21,27 +21,27 @@ all-features = true
 rustdoc-args = ["--generate-link-to-definition"]
 
 [features]
-default = ["protocol"]
-
-## Wire-protocol types (`HarnessMessage`, `InspectorCommand`, …) plus length-prefixed
-## MessagePack framing helpers. No `egui` dependency beyond `egui::accesskit`.
-protocol = ["dep:rmp-serde", "dep:serde", "dep:serde_bytes", "egui/serde"]
+default = []
 
 ## Cross-platform local-socket name helpers ([`transport::socket_name`],
 ## [`transport::generate_socket_target`]) built on `interprocess`: unix domain sockets on
 ## unix, named pipes on Windows. Shared by both ends of the connection.
 transport = ["dep:interprocess", "dep:tempfile"]
 
+## [`encode_png`] — shared screenshot PNG encoder so every peer (live plugin + kittest
+## harness) produces identically-encoded frames.
+png = ["dep:image", "image/png"]
+
 ## `InspectionPlugin` — an `egui::Plugin` impl that streams frames + accesskit tree to
 ## an inspector over a local socket and applies received commands. Auto-attaches when
 ## the [`INSPECTION_SOCKET_ENV_VAR`] env var is set.
-plugin = ["protocol", "transport", "dep:image"]
+plugin = ["transport", "png"]
 
 [dependencies]
-egui.workspace = true
-serde = { workspace = true, optional = true }
-serde_bytes = { version = "0.11.17", optional = true }
-rmp-serde = { workspace = true, optional = true }
+egui = { workspace = true, features = ["serde"] }
+serde.workspace = true
+serde_bytes = "0.11.17"
+rmp-serde.workspace = true
 image = { workspace = true, optional = true }
 interprocess = { version = "2.4", optional = true }
 tempfile = { workspace = true, optional = true }