tests: every test dir is removed when its test ends, pass or panic

Many test helpers removed their temp dir only before the test, or only
on pass, so runs left them behind: 117,000 filled /tmp (RAM here) and
stopped a mutation run. Each now holds a guard that removes the dir on
drop, unwind included; servers started on a test dir hold its guard
until they stop. A full cargo test --workspace leaves 0 dirs (859
before). just sweep-test-tmp stays for runs killed mid-test.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-store/src/viewer.rs

@@ -139,10 +139,20 @@ pub fn read(data_dir: &Path) -> Option<ViewerCommand> {
 mod tests {
     use super::*;
 
+    /// A test's dir, removed when the test ends, pass or panic.
+    struct TestDir(std::path::PathBuf);
+
+    impl Drop for TestDir {
+        fn drop(&mut self) {
+            let _ = std::fs::remove_dir_all(&self.0);
+        }
+    }
+
     #[test]
     fn requests_round_trip_and_count_up() {
         let dir = std::env::temp_dir().join(format!("wb-viewer-{}", std::process::id()));
         std::fs::create_dir_all(&dir).unwrap();
+        let _cleanup = TestDir(dir.clone());
         let a = request(&dir, ViewerTarget::Entry { entry_id: 7 }).unwrap();
         let b = request(
             &dir,