perf(streaming): optimize TTFT and reduce buffering latency

Core changes:
- stream.go: New accumulation buffer architecture with scanner goroutine
  + select loop, MinChars=16, MaxWait=10ms, first-flush-immediate
- dedupe.go: Add TrimContinuationOverlapFromBuilder to avoid string copies
- claude/stream_runtime_core.go: Integrate toolstream for incremental text
- claude/stream_runtime_finalize.go: toolstream flush support
- stream_emitter.js: Reduce DeltaCoalescer thresholds (160->16 chars, 80->20ms)
- empty_retry: Add thinking-aware empty output detection
- Fix reasoning_content leak and finish_reason=null in edge cases
- Fix tail content truncation when max_tokens exceeded

Tests: sync test expectations with upstream for thinking content

internal/js/chat-stream/stream_emitter.js

@@ -1,7 +1,7 @@
 'use strict';
 
-const MIN_DELTA_FLUSH_CHARS = 160;
-const MAX_DELTA_FLUSH_WAIT_MS = 80;
+const MIN_DELTA_FLUSH_CHARS = 16;
+const MAX_DELTA_FLUSH_WAIT_MS = 20;
 
 function createChatCompletionEmitter({ res, sessionID, created, model, isClosed }) {
   let firstChunkSent = false;