[sgl-router] Add SGLang-compatible DeepSeek V4.1 Flash rendering (#40532)

experimental/sgl-router/README.md

@@ -204,7 +204,7 @@ settings are not inferred from the router's environment. Disabling forwarding pr
 engine behavior but does not establish parity for local routing hashes.
 
 Also set `--disable-input-ids-forwarding` for array-only templates: Dynamo may wrap
-string content into arrays differently from the worker. Dynamo 5.1.2 does not expose
+string content into arrays differently from the worker. The pinned Dynamo renderer does not expose
 its conversion flag, so the router cannot automatically block these templates.
 Detailed content-format parity coverage follows in #39133.
 
@@ -221,6 +221,11 @@ official/preview effort profile is detected from the checkpoint's
 `config.json`, as in SGLang. Reference prompts live in `tests/fixtures/deepseek/`
 and are regenerated by `tests/scripts/generate_deepseek_parity.py`.
 
+V4.1 Flash uses Dynamo's separate V4.1 encoder with SGLang's numeric reasoning
+budgets, tool payloads, and `<｜System｜>` markers. Developer messages and media
+are left to the worker (the pinned encoder renders them differently), and a
+non-default `SGLANG_DSV41_REASONING_EFFORT` needs the forwarding precautions above.
+
 ## Kimi-K3
 
 Kimi-K3 renders through dynamo-render's native XTML formatter with SGLang's