asset slice + testing initiative: fixed assets, property/mutation/fuzz testing, docs

fuzz/fuzz_targets/fuzz_journal_line.rs

@@ -0,0 +1,24 @@
+// Fuzz target: journal line parsing must never panic, and the canonical
+// form must be a fixed point: parse(canonical(x)) re-serializes to the
+// same canonical bytes. Staged here; `cargo fuzz init` wires it up.
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use ledger_store::JournalLine;
+
+fuzz_target!(|data: &[u8]| {
+    // Garbage must return Err, never panic.
+    if let Ok(line) = serde_json::from_slice::<JournalLine>(data) {
+        let canon = line.to_canonical();
+        let line2: JournalLine =
+            serde_json::from_str(&canon).expect("canonical form must re-parse");
+        assert_eq!(
+            line2.to_canonical(),
+            canon,
+            "canonical form is not a fixed point"
+        );
+        // Sequence numbers survive the round trip.
+        assert_eq!(line.seq, line2.seq);
+        assert_eq!(line.line_type, line2.line_type);
+    }
+});