fix(perf): prevent memory leak and TextEncoder allocation on hot path

- Add performance.clearMarks/clearMeasures before creating new marks to
  prevent timeline accumulation in long-lived processes.
- Replace new TextEncoder().encode(str).length with Buffer.byteLength to
  avoid allocating a full Uint8Array just to measure byte length.

open-sse/handlers/chatCore/streamingPipeline.ts

@@ -72,6 +72,9 @@ export function assembleStreamingPipeline(
   },
   deps: StreamingPipelineDeps = DEFAULT_DEPS
 ) {
+  performance.clearMarks(PIPELINE_START);
+  performance.clearMarks(PIPELINE_END);
+  performance.clearMeasures(PIPELINE_MEASURE);
   performance.mark(PIPELINE_START);
   // ── Phase 9.3: Progress tracking (opt-in) ──
   const progressEnabled = deps.wantsProgress(args.clientRawRequestHeaders);

open-sse/utils/stream.ts

@@ -644,7 +644,7 @@ export function createSSEStream(options: StreamOptions = {}) {
   // tools can query performance.getEntriesByType("mark") filtered by name.
   let bodySize = 0;
   try {
-    bodySize = body ? new TextEncoder().encode(JSON.stringify(body)).length : 0;
+    bodySize = body ? Buffer.byteLength(JSON.stringify(body), "utf8") : 0;
   } catch {
     /* body may not be JSON-serialisable (e.g. FormData, Blob) — metric stays 0 */
   }