Fix concurrent checklist sync and consistent actions

tests/test_sync.js

@@ -0,0 +1,30 @@
+const assert = require("node:assert/strict");
+const sync = require("../static/sync.js");
+
+// A second client advances the shared revision while this client is saving.
+// The in-flight local value must not be replaced by the older server value.
+const local = { "crop-parsnip": true, "fish-eel": false };
+const remoteWhileSaving = {
+  revision: 6,
+  checks: { "crop-parsnip": false, "fish-eel": true }
+};
+const merged = sync.mergeSnapshot(local, new Set(["crop-parsnip"]), 5, remoteWhileSaving);
+assert.equal(merged.applied, true);
+assert.equal(merged.checks["crop-parsnip"], true);
+assert.equal(merged.checks["fish-eel"], true);
+
+// A response already superseded by a later write must never roll the client back.
+const stale = sync.mergeSnapshot(merged.checks, new Set(), 7, remoteWhileSaving);
+assert.equal(stale.applied, false);
+assert.equal(stale.revision, 7);
+
+// The write response is authoritative for its own item even after an interleaved poll.
+const confirmed = sync.confirmWrite(merged.checks, 6, {
+  itemId: "crop-parsnip",
+  checked: true,
+  revision: 7
+});
+assert.equal(confirmed.checks["crop-parsnip"], true);
+assert.equal(confirmed.revision, 7);
+
+console.log("sync race tests passed");