[Core] structured generation mode for DiffusionGemma model (Jev-like)

examples/features/structured_diffusion/README.md

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+# Structured reads on DiffusionGemma
+
+A discrete diffusion model denoises a whole canvas per forward pass. If the
+canvas is seeded with the answer's fixed text and only the answer slots are
+left as noise, one denoise step gives a distribution over each slot. These
+`extra_args` fields (`vllm_xargs` on the OpenAI server) expose that:
+
+| field | type | meaning |
+| --- | --- | --- |
+| `diffusion_seed_canvas` | `list[int]`, exactly `canvas_length` ids | replaces the random initial canvas after prefill |
+| `diffusion_pinned` | `list[int]` of canvas positions | held at their seed value on every denoise step, so a read past one step keeps its template |
+| `diffusion_max_steps` | `int` | denoise steps before the canvas is emitted |
+| `diffusion_read_only` | `bool` | emit the argmax canvas as soon as the cap is reached, end the request there, and return temperature-1 logprobs at every position |
+
+`structured_server.py` turns a question schema into those fields. It serves
+`/v1/chat/completions`: the system message is the schema, the user message
+is the state JSON, and the reply content is one distribution per question
+with a standard error over a few noise draws.
+
+```bash
+vllm serve google/diffusiongemma-26B-A4B-it \
+    --diffusion-config '{"canvas_length": 64}' --max-logprobs 32 --enable-prefix-caching
+python examples/features/structured_diffusion/structured_server.py \
+    --upstream http://127.0.0.1:8000 --tokenizer google/diffusiongemma-26B-A4B-it --canvas 64
+curl -s localhost:8011/v1/chat/completions -H 'content-type: application/json' -d '{
+  "messages": [
+    {"role": "system", "content": "{\"questions\": [{\"id\": \"urgent\", \"type\": \"noul\", \"instructions\": \"Does the customer need a reply within the hour?\"}]}"},
+    {"role": "user", "content": "{\"ticket\": \"Everything is down and we have a demo at noon.\"}"}
+  ]}'
+```
+
+The attention backend is picked as for Gemma 4: FlashAttention 4 on every
+layer when available, otherwise Triton. FlashInfer cannot serve this model (a
+batch mixes causal prefill with bidirectional denoising), and
+`--attention-backend FLASHINFER` is rejected.
+
+Per-request canvas widths smaller than the served canvas require async
+scheduling. The diffusion async scheduler is selected automatically; no
+`--scheduler-cls` argument is needed. Synchronous execution supports full-width
+canvases only.
+
+Question types: `noul` (yes/no), `choice` with `options`, `score` with
+ordered `levels`. Each label must be a single token in the answer template,
+which the server checks with the tokenizer when a request uses the schema.
+
+`POST /v1/systemone` implements the Jev decision API. The body holds
+`state`, `questions` (a map of id to `type`, `instructions` and `criteria`)
+and `model`. Answers come back in that API's shapes: a `noul` probability,
+a `choice` with `probabilities` and `confidence`, or a `score` with a
+0-indexed `legend`. The schema options above go in the same body as
+extensions.
+
+A question may declare `depends_on` (read in a later stage with those
+answers in its prompt), `ask_if` (asked only when a named question's answer
+is among the listed ones, otherwise null) and `alone` (a read of its own).
+
+Images attach as `multipart/form-data`, with the JSON in a part named
+`request` and each image as a file part, or as an `images` array of data
+URLs.
+
+```bash
+curl -s localhost:8011/v1/systemone -H 'content-type: application/json' -d '{
+  "model": "jev-latest",
+  "state": {"ticket": "Everything is down and we have a demo at noon."},
+  "questions": {"urgent": {"type": "noul", "instructions": "Does the customer need a reply within the hour?"}}}'
+```
+
+`"think": N` in the schema lets the model write up to N tokens in its
+thought channel before the read. The thought is an ordinary generation with
+the chat template's thinking marker on, and the read then runs with the
+thought in its prompt, so the answer slots condition on it. The noise draws
+of a decision share one thought. `diagnostics.thought` returns the text, its
+length in tokens, whether the model closed the channel itself and the
+generation time.