test: red for #83 — Registry::load must read a legacy pre-0047 bare-float params line

RED test pinning the real defect of bug F5 (surfaced by the milestone
"The World, part II" fieldtest). Registry::load cannot deserialize a
runs.jsonl line written before the cycle-0047 typed-Scalar params
migration, where each param value is a bare JSON float (e.g.
["sma_cross.fast",2.0]) rather than the tagged {"F64":2.0}. The test
feeds a minimal autonomous legacy line and asserts it loads — the bare
float read back via the documented f64 coercion — instead of erroring at
the first param.

Fails RED at the diagnosed cause (Parse "expected value" on the bare
float). GREEN side (back-compat read, no change to the typed-Scalar
forward format) follows.

refs #83

crates/aura-registry/src/lib.rs

@@ -236,6 +236,31 @@ mod tests {
         let _ = fs::remove_file(&path);
     }
 
+    /// A `runs.jsonl` line written before the cycle-0047 typed-`Scalar` params
+    /// migration carries each param value as a **bare JSON number** (`2.0`),
+    /// where the current format tags it (`{"F64":2.0}`). `load()` is the durable
+    /// C18 read-path and must stay back-compatible across that wire-shape change:
+    /// a legacy bare-float line must still load, the bare float read back as the
+    /// documented `f64` coercion — not erroring at the first param. (The current
+    /// typed shape is already covered by the round-trip test.)
+    #[test]
+    fn load_reads_a_legacy_bare_float_params_line() {
+        let path = temp_path("legacy_bare_float");
+        let _ = fs::remove_file(&path);
+        // An autonomous, minimal legacy line: a single bare-float param is the
+        // whole trigger (the rest of the RunReport is in the current shape). This
+        // mirrors the real pre-0047 store, where the first param `2.0` is where
+        // the typed-Scalar deserializer first chokes.
+        let legacy = r#"{"manifest":{"commit":"c","params":[["sma_cross.fast",2.0]],"window":[1,2],"seed":0,"broker":"b"},"metrics":{"total_pips":0.0,"max_drawdown":0.0,"exposure_sign_flips":0}}"#;
+        fs::write(&path, format!("{legacy}\n")).expect("write legacy line");
+        let reg = Registry::open(&path);
+        let reports = reg.load().expect("a legacy bare-float params line must still load");
+        assert_eq!(reports.len(), 1);
+        // the bare float reads back as the documented f64 coercion
+        assert_eq!(reports[0].manifest.params, vec![("sma_cross.fast".to_string(), Scalar::f64(2.0))]);
+        let _ = fs::remove_file(&path);
+    }
+
     #[test]
     fn rank_by_orders_best_first_per_metric() {
         let reports = vec![