feat: add LTX-2.5 support

[pwilkin]

I'll let Claude describe the details, it's surprisingly not that different from LTX-2.3:

LTX-2.5 shares its video VAE, audio VAE, vocoder and spatial upscaler architecture with LTX-2.3, so those need no changes. The transformer differs in two places and the text encoder moves from Gemma 3 to Gemma 4.

Transformer:

• Detect ff_bias and audio_ff_bias separately from the weights. LTX-2.5 sets ff_bias=false but leaves audio_ff_bias at its default, so a single flag would drop the audio FFN biases.
• Add the keyframes_abs_pos_embedding marker, applied after patchify_proj to the tokens of the first latent frame.
• Cast learnable_registers to F32 when the checkpoint stores them at another type, as the published LTX-2.5 GGUFs do.

Text encoder (LLMArch::GEMMA4_12B):

• Full-attention layers use a 512-wide head with a single KV head and no v_proj, taking V from the raw K projection; sliding layers keep 256/8.
• V is normalized with a weightless RMS norm and never rotated.
• Attention runs unscaled, and RoPE is proportional on full-attention layers, implemented with freq_factors so the unrotated pairs collapse to identity without reordering the head.
• Each layer ends with its learned layer_scalar.
• Both the transformers and the llama.cpp GGUF layer-norm namings are accepted, so the bundled single-file checkpoint loads directly.

The Gemma 4 vocabulary differs from Gemma 3 only in reserved slots and shares its merge table, so the embedded Gemma vocab still produces the right token ids.

LTX-2.5 bundles the text projection into its text encoder, so --embeddings-connectors is no longer required; the projection is looked up under the --llm prefix as well.

Verified on the Q8_0 dev transformer for T2V, I2V, spatial latent upscaling and a Q8_0-converted text encoder, and against Z-Image for the shared attention and feed-forward changes.

Summary

It works. That's good, right? :) The guys from the LTX team said they'd gladly help if there are any issues (they contacted me by mail after I filled in the gating form on HF).

Checklist

• I have read and confirmed this PR follows the contribution guidelines.