fuzz campaigns + gate: wave 7 close — three 15-min detached campaigns clean (payload_codegen finding harness-model: serde_json default f64 parse drifts 1-2 ULPs; invariant pinned to 2-ULP float-leaf drift + decode determinism, crash input promoted as payload_codec/seed-900), fuzz/.gitignore retightened to seed-* tracking, corpus-replay gate joins AGENTS.md verification list, research doc §9 implementation log, README finalized

fuzz/shared/src/payload_codec.rs

@@ -69,13 +69,23 @@ pub fn fuzz_payload_codec(data: &[u8]) {
     assert_decode_partition::<Vec<Value>>(data);
 
     // A successfully-decoded `Value` re-encodes byte-stably: the Value
-    // round-trip closes over the raw input, with serde_json's
-    // canonicalization (duplicate-key last-write-wins, number-to-NormalizedNumber,
-    // key ordering) pinned — not re-derived, just not regressed.
+    // round-trip closes over the raw input up to serde_json's
+    // canonicalization (duplicate-key last-write-wins, key ordering) —
+    // pinned, not re-derived. serde_json's default fast-path f64
+    // parser is not exactly correctly-rounded (its parse of its own
+    // ryu serialization may drift 1–2 ULPs; the ADR-020 §4 posture
+    // pins default serde_json, not the `float_roundtrip` flavor), so
+    // float leaves are compared under the pinned drift bound; every
+    // other shape must be exactly equal.
     if let Ok(v) = payload_as::<Value>(data) {
         let bytes = encode_payload(&v).expect("a decoded `Value` must re-encode");
         let back: Value = payload_as(&bytes).expect("the encoded `Value` must decode");
-        assert_eq!(back, v, "the `Value` round-trip must close over raw input");
+        assert_canonicalized_closure(&v, &back);
+        let back_again: Value = payload_as(&bytes).expect("the encoded `Value` must decode");
+        assert_eq!(
+            back, back_again,
+            "payload decode must be deterministic over stored bytes"
+        );
     }
 
     assert_deep_input_loudness();
@@ -92,6 +102,68 @@ fn assert_decode_partition<T: serde::de::DeserializeOwned>(data: &[u8]) {
     }
 }
 
+/// serde_json's default fast-path f64 parse is not exactly
+/// correctly-rounded: parsing its own ryu serialization can drift by
+/// at most this many ULPs (one canonicalization step; measured over
+/// randomized campaigns — see the research doc's implementation log).
+const FLOAT_DRIFT_ULPS: f64 = 2.0;
+
+fn assert_canonicalized_closure(v: &Value, back: &Value) {
+    match (v, back) {
+        (Value::Number(a), Value::Number(b)) => {
+            if a.is_f64() && b.is_f64() {
+                let x = a.as_f64().expect("checked is_f64");
+                let y = b.as_f64().expect("checked is_f64");
+                if y == x {
+                    return;
+                }
+                let drift = (y - x).abs() / canonicalization_ulp(x);
+                assert!(
+                    drift <= FLOAT_DRIFT_ULPS,
+                    "float canonicalization drifted {drift} ULPs, over the \
+                     {FLOAT_DRIFT_ULPS}-ULP bound: {x} -> {y}"
+                );
+            } else {
+                assert_eq!(a, b, "an integer leaf lost exactness across the round-trip");
+            }
+        }
+        (Value::Array(a), Value::Array(b)) => {
+            assert_eq!(
+                a.len(),
+                b.len(),
+                "array length changed across the round-trip"
+            );
+            for (x, y) in a.iter().zip(b.iter()) {
+                assert_canonicalized_closure(x, y);
+            }
+        }
+        (Value::Object(a), Value::Object(b)) => {
+            assert_eq!(
+                a.len(),
+                b.len(),
+                "object key set changed across the round-trip"
+            );
+            for (key, x) in a {
+                let y = b
+                    .get(key)
+                    .unwrap_or_else(|| panic!("key {key:?} lost across the round-trip"));
+                assert_canonicalized_closure(x, y);
+            }
+        }
+        _ => assert_eq!(v, back, "the `Value` round-trip must close over raw input"),
+    }
+}
+
+fn canonicalization_ulp(x: f64) -> f64 {
+    let a = x.abs();
+    let up = a.next_up();
+    if up > a {
+        up - a
+    } else {
+        a - a.next_down()
+    }
+}
+
 /// Deep inputs decode loud and typed: past serde_json's recursion
 /// limit, deserialization fails with `Error::Codec` — never a stack
 /// overflow, never an untyped panic. The encode side handles deep