model : add Tencent Hy 4 (hy_v4) preview architecture support (#28127)

* model: add Tencent Hy 4 (hy_v4) preview architecture support

Adds support for the Tencent Hy 4 model (Hugging Face architecture
HYV4ForCausalLM, GGUF arch hy_v4):

Add HF -> GGUF conversion script (conversion/hy_v4.py) and wire it into the conversion registry
Register hy_v4 GGUF constants, arch enum, and writer support
Implement the hy-v4 model graph, hparams, vocab and context changes
Register the new arch in llama-arch and models registry
Extend arch tests to cover hy_v4

Assisted by Claude Opus 5

* Update convert_hf_to_gguf_update.py

Co-authored-by: fairydreaming <166155368+fairydreaming@users.noreply.github.com>

* Update conversion/base.py

Co-authored-by: fairydreaming <166155368+fairydreaming@users.noreply.github.com>

* convert : move hy_v4 entry to the same place as in convert_hf_to_gguf_update.py

* model : apply changes related to n_ff_exp becoming per-layer in Hy4-preview

* n_layer_all

---------

Co-authored-by: fairydreaming <166155368+fairydreaming@users.noreply.github.com>
Co-authored-by: Stanisław Szymczyk <sszymczy@gmail.com>
Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@huggingface.co>

conversion/base.py

@@ -1507,6 +1507,9 @@ class TextModel(ModelBase):
         if chkhsh == "bba3b3366b646dbdded5dbc42d59598b849371afc42f7beafa914afaa5b70aa6":
             # ref: https://huggingface.co/tencent/Hunyuan-4B-Instruct
             res = "hunyuan-dense"
+        if chkhsh == "e6ddf9c6686791c12d698d34c31ab9be1fea9af5a3d9a6909783ab382198ae1c":
+            # ref: https://huggingface.co/tencent/Hy4-preview
+            res = "hy_v4"
         if chkhsh == "a6b57017d60e6edb4d88ecc2845188e0eb333a70357e45dcc9b53964a73bbae6":
             # ref: https://huggingface.co/tiiuae/Falcon-H1-0.5B-Base
             res = "falcon-h1"