feat(space): add a Hugging Face Space demo

[FenjuFu]

Summary

Adds spaces/domux-demo, a Gradio demo that can run as a Hugging Face Space so people can try Domux without deploying it, as #10 asks (in the spirit of baidu/Unlimited-OCR).

• Type a command, or pick one of 8 examples taken from the README and the four test-set categories. The demo shows the parsed action|device|attribute|value|unit|room|floor slots as a table, the raw model output, whether every line has 7 fields (same rule as eval/run_eval.py, including the & separator), and the latency.
• Two backends:
  • Local model (default). Loads iFlytekOpenSource/Domux with transformers in BF16, pinned to revision 6c71a32f. This mirrors cases/domux-4057-full-eval/eval_direct.py (transformers 5.15.1, AutoModelForCausalLM, greedy decoding, 256 new tokens), which scored 97.58% on the full test set. On a ZeroGPU Space each request is wrapped in @spaces.GPU.
  • OpenAI-compatible endpoint. Used when DOMUX_API_BASE is set. It calls a vLLM or SGLang server with the same request as eval/run_eval.py, so the Space can run on free CPU hardware.
• .github/workflows/sync-space.yml uploads the directory to the Space on changes to main or on manual dispatch. It targets iFlytekOpenSource/Domux-Demo unless the HF_SPACE_ID variable says otherwise, and skips with a notice when the HF_TOKEN secret is not configured.
• .github/workflows/space-check.yml runs the demo tests on PRs.
• A pointer is added in the Demo section of both READMEs, plus an Unreleased CHANGELOG entry.

Closes #10

Maintainer setup

1. Create the Space (e.g. iFlytekOpenSource/Domux-Demo) and pick ZeroGPU hardware, or CPU with DOMUX_API_BASE.
2. Add the HF_TOKEN secret (and HF_SPACE_ID if the name differs).
3. Run Sync Hugging Face Space once.

Testing

• pytest spaces/domux-demo/tests: 9 passed. Covers slot splitting and format flags, input limits, the endpoint request shape, backend selection, and building the Gradio app with gradio 6.28.0.
• Ran app.py locally against a stub OpenAI-compatible server and used the UI in a browser. The multi-room example rendered as 4 slot rows with a valid-format badge, and an empty submit shows the validation message.
• The Space card (spaces/domux-demo/README.md) parses with huggingface_hub.SpaceCard, and zero-a10g is a valid SpaceHardware value.
• Not tested: the local GPU path. I have no GPU here, so it has not been run end to end. It follows the loading and generation code of cases/domux-4057-full-eval line for line, and the first Space build will exercise it.