fix(streaming): complete interim anchor prose

Treat status-less interim assistant events as complete semantic boundaries so streaming Markdown flushes its final lookahead character before tool activity. Add behavior coverage for the real interim-to-tool Anchor pipeline.

Assisted-by: Hermes Agent:gpt-5.6-sol
Assisted-by: Codex:gpt-5.3-codex-spark

static/assistant_turn_anchors.js

@@ -302,6 +302,7 @@
         ?normalizeAssistantTurnAnchorTerminalState(explicit,sourceType)||explicit
         :explicit;
     }
+    if(sourceType==='interim_assistant') return 'completed'; // interim prose is a complete semantic boundary, unlike token deltas.
     if(kind==='tool_started') return 'running';
     if(kind==='tool_completed') return _own(payload,'is_error')?'error':'completed';
     if(kind==='terminal_status'){