review: docs section, 1 MiB grammar pre-check, negative tests, measured compile overhead

Addresses the #192 review: server usage documented in docs/grammar-draft.md
(incl. back-compat statement for the additive SUBMIT field and the #100-class
near-tie caveat); gateway pre-checks grammar payloads at 1 MiB (matching the
engine's gbytes bound); negative tests for non-dict response_format, empty and
oversized grammars, plus an explicit test that malformed GBNF passes the
gateway by design (engine fail-soft, draft-source semantics). Measured compile
overhead: 7.8 us/request typical schema, 17.9 us at the 32-level nesting cap.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

c/openai_server.py

@@ -468,6 +468,9 @@ def generation_options(body, limit):
             raise APIError(400, "`response_format.type` must be \"text\", \"json_object\", "
                                 "\"json_schema\" or \"gbnf\".",
                            "response_format", "unsupported_value")
+        if grammar is not None and len(grammar.encode("utf-8")) > (1 << 20):
+            raise APIError(400, "`response_format` grammar/schema exceeds 1 MiB.",
+                           "response_format", "invalid_value")
 
     maximum = body.get("max_completion_tokens")
     maximum_param = "max_completion_tokens"