Remove core protocol dependency [1/2] (#20324)

## Why

This stack moves `codex-tui` away from the core protocol event surface
and toward app-server API shapes plus TUI-owned local models. This first
PR sets up the lower-risk foundation: it introduces the local model
surface and extracts app-server event routing into focused TUI modules
while preserving the existing behavior for the larger migration in PR2.

This PR is part 1 of a 2-PR stack:

1. Add TUI-owned replacement models and extract app-server event
routing.
2. Move the active TUI flow to app-server notifications and delete
obsolete adapter code.

## What changed

- Added TUI-owned approval, diff, session state, session resume, token
usage, and user-message models.
- Added `app/app_server_event_targets.rs` and `app/app_server_events.rs`
to hold app-server event targeting and dispatch logic outside `app.rs`.
- Updated app/status tests to use the local model layer and added
focused routing coverage.
- Boxed a few large async TUI test futures so this base layer remains
checkable without overflowing the default test stack.

## Verification

- `cargo check -p codex-tui --tests`

codex-rs/tui/src/diff_model.rs

@@ -0,0 +1,21 @@
+//! Minimal file-change model used by TUI diff rendering and approval previews.
+
+use std::path::PathBuf;
+
+use serde::Deserialize;
+use serde::Serialize;
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(tag = "type", rename_all = "snake_case")]
+pub(crate) enum FileChange {
+    Add {
+        content: String,
+    },
+    Delete {
+        content: String,
+    },
+    Update {
+        unified_diff: String,
+        move_path: Option<PathBuf>,
+    },
+}