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-mcp/src/tin_files.rs

@@ -357,11 +357,27 @@ fn nul_interleaved(bytes: &[u8]) -> bool {
 mod tests {
     use super::*;
 
-    fn dir(tag: &str) -> std::path::PathBuf {
+    /// A test's dir, removed when the test ends, pass or panic.
+    struct TestDir(std::path::PathBuf);
+
+    impl std::ops::Deref for TestDir {
+        type Target = std::path::Path;
+        fn deref(&self) -> &std::path::Path {
+            &self.0
+        }
+    }
+
+    impl Drop for TestDir {
+        fn drop(&mut self) {
+            let _ = std::fs::remove_dir_all(&self.0);
+        }
+    }
+
+    fn dir(tag: &str) -> TestDir {
         let d =
             std::env::temp_dir().join(format!("ledger-mcp-tin-files-{}-{tag}", std::process::id()));
         let _ = std::fs::remove_dir_all(&d);
-        d
+        TestDir(d)
     }
 
     /// INVARIANTS 1-2: a recorded file is refused from then on, by this
@@ -382,7 +398,6 @@ mod tests {
         assert_eq!(text.lines().count(), 1);
         // Only the hash is kept.
         assert!(!text.contains("PNG"));
-        let _ = std::fs::remove_dir_all(&vault);
     }
 
     /// The registry's folder for a check (it never exists) and no vault.
@@ -614,7 +629,6 @@ Date,Description,Amount,Running Bal.,Reference
         .expect_err("a registry that can't be read let a file in");
         assert!(why.contains("couldn't check the file"), "{why}");
         assert!(record(&vault, b"bytes").is_err());
-        let _ = std::fs::remove_dir_all(&vault);
     }
 
     /// INVARIANT 4: text holding more than two TIN-shaped numbers ending