Add a TurnDiffTracker to create a unified diff for an entire turn (#1770)

This lets us show an accumulating diff across all patches in a turn.
Refer to the docs for TurnDiffTracker for implementation details.

There are multiple ways this could have been done and this felt like the
right tradeoff between reliability and completeness:
*Pros*
* It will pick up all changes to files that the model touched including
if they prettier or another command that updates them.
* It will not pick up changes made by the user or other agents to files
it didn't modify.

*Cons*
* It will pick up changes that the user made to a file that the model
also touched
* It will not pick up changes to codegen or files that were not modified
with apply_patch

codex-rs/core/Cargo.toml

@@ -34,6 +34,7 @@ serde_json = "1"
 serde_bytes = "0.11"
 sha1 = "0.10.6"
 shlex = "1.3.0"
+similar = "2.7.0"
 strum_macros = "0.27.2"
 thiserror = "2.0.12"
 time = { version = "0.3", features = ["formatting", "local-offset", "macros"] }