split: sync-engine rewrite (sync-operations + sync-client.ts)

Replace the single unrestricted-syncer.ts with a two-loop architecture:
- syncer.ts drains the FIFO wire queue (HTTP + WS handlers).
- reconciler.ts moves files to make localPath match remoteRelativePath
  (topo-sorted move graph, in-memory cycle resolution with crash-safe
  swap markers).
- sync-event-queue.ts holds the byDocId / byLocalPath indexes and the
  pending-create promise chain.
- offline-change-detector.ts, expected-fs-events.ts, types.ts, and a
  rewritten cursor-tracker.ts / file-change-notifier.ts round it out.
Plus sync-client.ts wiring, tracing/sync-history.ts updates, index.ts
re-exports, and sync-client tsconfig/webpack/package.json.

frontend/sync-client/webpack.config.js

@@ -49,11 +49,6 @@ module.exports = [
                 type: "umd"
             },
             globalObject: "this"
-        },
-        resolve: {
-            fallback: {
-                ws: false // Exclude `ws` from the browser bundle
-            }
         }
     }),
     merge(common, {
@@ -62,10 +57,6 @@ module.exports = [
             path: path.resolve(__dirname, "dist"),
             filename: "sync-client.node.js",
             libraryTarget: "commonjs2"
-        },
-        externals: {
-            bufferutil: "bufferutil",
-            "utf-8-validate": "utf-8-validate" // required for ws: https://github.com/websockets/ws/issues/2245#issuecomment-2250318733
         }
     })
 ];