test: add canonical append-log example (#23)

Validates the FIRST API with a minimal crash-consistency test:
- Append-only log with 2 writes + fsync
- 3 explicit crash points
- Prefix-consistency invariant check

Closes #16

Author: siphonite

tests/canonical_append_log.rs

@@ -0,0 +1,56 @@
+//! Canonical example: Append-only log with crash-consistency verification.
+//!
+//! This test validates the FIRST API by performing a simple append-log
+//! workload with explicit crash points and invariant checking.
+
+use std::fs::File;
+use std::io::{Read, Write};
+
+#[test]
+fn append_log_atomicity() {
+    first::test()
+        .run(|env| {
+            let path = env.path("append.log");
+
+            // Write record 1
+            let mut file = File::create(&path).unwrap();
+            file.write_all(b"RECORD1\n").unwrap();
+            first::crash_point("after_write_1");
+
+            // Write record 2
+            file.write_all(b"RECORD2\n").unwrap();
+            first::crash_point("after_write_2");
+
+            // Fsync to make durable
+            file.sync_all().unwrap();
+            first::crash_point("after_fsync");
+        })
+        .verify(|env, crash_info| {
+            let path = env.path("append.log");
+
+            // Read whatever survived
+            let mut contents = String::new();
+            if let Ok(mut f) = File::open(&path) {
+                f.read_to_string(&mut contents).ok();
+            }
+
+            let records: Vec<_> = contents.lines().collect();
+
+            // INVARIANT: Records are prefix-consistent
+            // Either: [], ["RECORD1"], or ["RECORD1", "RECORD2"]
+            // Never: ["RECORD2"] alone (would violate append-only semantics)
+
+            match records.as_slice() {
+                [] => { /* Nothing persisted - fine */ }
+                ["RECORD1"] => { /* Partial - fine */ }
+                ["RECORD1", "RECORD2"] => { /* Complete - fine */ }
+                other => {
+                    panic!(
+                        "Invariant violation at '{}': unexpected log state {:?}",
+                        crash_info.label, other
+                    );
+                }
+            }
+        })
+        .execute();
+}