kv-sync: refuse per-item replays (seq/restores in the item plaintext)

A malicious server could re-serve an old but valid envelope of an item under
a new, higher revision: the AAD does not bind the revision and the Merkle
root was only checked when head_revision == cursor. The client took it as
the latest version, rolled the item back and signed over it in its next
manifest.

* kv-model Item: `seq: u64` (serde default 0) and `restores: Option<u64>`.
* kv-sync replay module: pure decision rules (check_successor,
  next_version, stamp, same_content). Every pulled live record must
  authenticate and have seq > the base's seq, unless it carries
  restores = t with base_revision < t < revision (edit beat a delete this
  device never saw) or is identical to the base. Otherwise: Integrity alarm.
  Tombstones and items without a base pass.
* engine: check on every pulled record; edit-beats-delete re-seals with
  restores = tombstone revision; merges write seq = max + 1 and adopt the
  remote version when only seq/restores differ. Public
  engine::next_local_version gives the seq/restores for a local save
  (used by kv-ffi save_item and the tests).
* verify_manifest checks the Merkle root whenever no held item is newer
  than head_revision (previously only at head == cursor).
* Tests: replay alarm (clean and pending receivers, plus kv-ffi), harmless
  replay of the current version, tampered envelope, concurrent equal-seq
  edits across three devices, edit-beats-delete with a device that skipped
  the tombstone and a later replay of the restore, legacy items without seq,
  hidden item with cursor past the manifest head, and the shared
  item_replay.json decision table.
* gen-vectors: envelope vector item payload now includes seq.

FFI surface unchanged.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/kv-crypto/tests/vectors/envelope.json

@@ -7,10 +7,10 @@
   "vectors": [
     {
       "aad": "6b657374726f76652f76312f6974656d000101010101014101810101010101010100020202020202420282020202020202020003030303030343038303030303030303",
-      "ciphertext": "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",
+      "ciphertext": "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",
       "envelope": {
         "alg": "xc20p",
-        "ct": "CugkthsiU_QAFdIvUu50EuW-lc8XGFvYSQ4OdyANULX2Kcbf0uK8zB2GoGxkUxUKluHI_IHxqCtIMPUR6DGqZkONNe2cfW0R2QSz1TZB8FF1Bm9jwNmF6pFzjvzXr7aiBbSoRx_IJ6EcUctZjkKSJ-8sMCDM3XSfJUU4ylQp6V9wtyWNh6a1IBiXZpWhW2535sR6SWCfppH2AtvrShXqc8h5CccmfXv4TeMXYkgP6rC5ZVPC1ARrXtAMiZ4wF1-9czPp2gEynYmjFYpxWoWoPYG4oBBePCzdD_svUuGXHQx6wcV1BP6idLiD_Utl2udX11jLPs14KOft1Es1y7pFImEDnkNr7rOuuheNN5pUq_k",
+        "ct": "EugkthsiU_QAFdIvUu50EuW-lc8XGFvYSQ4OdyANULX2Kcbf0uK8zB2GoGxkUxUKluHI_IHxqCtIMPUR6DGqZkONNe2cfW0R2QSz1TZB8FF1Bm9jwNmF6pFzjvzXr7aiBbSoRx_IJ6EcUctZjkKSJ74OQ0W9_06uWEU4ylQp6V9wtyWNh6a1IBiXZpWhW2535sR6SWCfppH2AtvrShXqc8h5CccmfXv4TeMXYkgP6rC5ZVPC1ARrXtAMiZ4wF1-9czPp2gEynYmjFYpxWoWoPYG4oBBePCzdD_svUuGXHQx6wcV1BP6idLiD_Utl2udX11jLPs14KOft1Es1y7pFIqRfiBNDiMh4t3ZZ-1p6zjc",
         "kid": "04040404-0404-4404-8404-040404040404",
         "n": "AQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEB",
         "v": 1
@@ -19,7 +19,7 @@
       "kid": "04040404-0404-4404-8404-040404040404",
       "name": "item payload (padded)",
       "nonce": "010101010101010101010101010101010101010101010101",
-      "plaintext": "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"
+      "plaintext": "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"
     },
     {
       "aad": "6b657374726f76652f76312f6974656d2d6b6579000101010101014101810101010101010100020202020202420282020202020202020003030303030343038303030303030303",

crates/kv-crypto/vectors/gen.rs

@@ -88,7 +88,7 @@ fn envelope_vectors() -> Value {
     let vault = id(0x02);
     let item = id(0x03);
     let key = SymKey::from_bytes(id(0x04), [0x42; 32]);
-    let item_json = br#"{"id":"03030303-0303-4303-8303-030303030303","kind":"login","title":"Example","created_at":1,"modified_at":1}"#;
+    let item_json = br#"{"id":"03030303-0303-4303-8303-030303030303","kind":"login","title":"Example","created_at":1,"modified_at":1,"seq":1}"#;
     type Case = (&'static str, ObjectType, Option<Uuid>, Option<Uuid>, Vec<u8>, [u8; 24]);
     let cases: Vec<Case> = vec![
         ("item payload (padded)", ObjectType::Item, Some(vault), Some(item), pad(item_json).to_vec(), [0x01; 24]),