fix(heretic2-nvfp4): parse tool_call arguments string->dict for Qwen3.6 template

render-verify caught it: Dvalin's calib tool_calls carry OpenAI wire-form JSON
string arguments, but the Qwen3.6 chat template does .items() on arguments (needs
a dict) → jinja TypeError. Parse string->dict in render_verify + the quant's
load_calib_chat. Confirmed: renders the exact qwen3_coder XML the seat emits
(prefixed bifrost.soong-lab.*, v0.3.13 generate_portrait, <think>, <tool_response>).

services/heretic2-nvfp4-quant/quant_nvfp4.py

@@ -61,6 +61,14 @@ def load_calib_chat(path, tokenizer, seqlen):
     rows = [json.loads(l) for l in open(path) if l.strip()]
     out = []
     for r in rows:
+        # Tool_call arguments arrive as OpenAI wire-form JSON *strings*; the Qwen3.6
+        # template does .items() on them → needs a dict. Parse string→dict so the
+        # forward-pass sees the qwen3_coder XML the seat emits. (render_verify finding.)
+        for m in r["messages"]:
+            for tc in (m.get("tool_calls") or []):
+                a = tc.get("function", {}).get("arguments")
+                if isinstance(a, str):
+                    tc["function"]["arguments"] = json.loads(a)
         text = tokenizer.apply_chat_template(
             r["messages"], tools=r.get("tools"),
             tokenize=False, add_generation_prompt=False, enable_thinking=True,