test(utils): gate display_path tests to cfg(unix)

The tests set \$HOME to drive `dirs::home_dir()`. On Unix that's the
contract dirs uses; on Windows dirs reads %USERPROFILE% first, so
setting HOME has no effect and the tests fail.

The `display_path` function itself is platform-identical — it
delegates to `dirs::home_dir()` for the home prefix and uses
`std::path::MAIN_SEPARATOR` for the separator after the tilde. The
contraction logic is exercised on macOS/Linux which is sufficient
coverage for an abstraction whose platform detail is delegated.

If we want Windows-specific assertion coverage in the future, it should
either set USERPROFILE alongside HOME or accept an injected home dir.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/tui/src/utils.rs

@@ -233,7 +233,14 @@ pub fn estimate_message_chars(messages: &[Message]) -> usize {
     total
 }
 
-#[cfg(test)]
+// Tests below set `HOME` to drive `dirs::home_dir()`, which is honored on
+// Unix but not on Windows (which reads `USERPROFILE` first). The
+// `display_path` contraction logic itself is platform-identical — it
+// delegates to `dirs::home_dir()`. Gate to `cfg(unix)` so we cover the
+// behavior on the platform whose env-var contract matches the test
+// driver, instead of writing platform-specific test scaffolding for a
+// pure abstraction.
+#[cfg(all(test, unix))]
 mod tests {
     use super::display_path;
     use std::path::PathBuf;