refactor: require external Gemma 2 and GPT-OSS tokenizers (#1974)

docs/lens.md

@@ -12,13 +12,17 @@ Lens uses a Lens diffusion transformer, the FLUX.2 VAE, and GPT-OSS-20B as the L
     - safetensors: https://huggingface.co/black-forest-labs/FLUX.2-dev/tree/main
 - Download GPT-OSS-20B
     - gguf: https://huggingface.co/unsloth/gpt-oss-20b-GGUF/tree/main
+- Download GPT-OSS-20B tokenizer.json
+    - https://huggingface.co/openai/gpt-oss-20b/tree/main
+
+Lens and Lens Turbo require an external GPT-OSS `tokenizer.json` matching the text encoder checkpoint. Save it as `tokenizer_gpt_oss.json` and pass it with `--tokenizer`; the tokenizer is not embedded in sd.cpp. See [JSON tokenizers](tokenizers.md) for CLI and C API usage.
 
 ## Examples
 
 ### Lens
 
 ```
-.\bin\Release\sd-cli.exe --diffusion-model ..\models\diffusion_models\lens_bf16.safetensors --llm "..\models\text_encoders\gpt-oss-20b-UD-Q8_K_XL.gguf" --vae ..\models\vae\flux2_ae.safetensors --cfg-scale 5.0  -p "A crystal dragon soaring through an aurora borealis sky, its entire body made of transparent faceted crystal refracting the green and purple aurora light into rainbow spectra, ice particles trailing from its wings, high fantasy digital art" --diffusion-fa -v
+.\bin\Release\sd-cli.exe --diffusion-model ..\models\diffusion_models\lens_bf16.safetensors --llm "..\models\text_encoders\gpt-oss-20b-UD-Q8_K_XL.gguf" --tokenizer ..\models\tokenizers\tokenizer_gpt_oss.json --vae ..\models\vae\flux2_ae.safetensors --cfg-scale 5.0  -p "A crystal dragon soaring through an aurora borealis sky, its entire body made of transparent faceted crystal refracting the green and purple aurora light into rainbow spectra, ice particles trailing from its wings, high fantasy digital art" --diffusion-fa -v
 ```
 
 <img width="256" alt="Lens example" src="../assets/lens/example.png" />
@@ -26,7 +30,7 @@ Lens uses a Lens diffusion transformer, the FLUX.2 VAE, and GPT-OSS-20B as the L
 ### Lens Turbo
 
 ```
-.\bin\Release\sd-cli.exe --diffusion-model ..\models\diffusion_models\lens_turbo_bf16.safetensors --llm "..\models\text_encoders\gpt-oss-20b-UD-Q8_K_XL.gguf" --vae ..\models\vae\flux2_ae.safetensors --cfg-scale 1.0  -p "A crystal dragon soaring through an aurora borealis sky, its entire body made of transparent faceted crystal refracting the green and purple aurora light into rainbow spectra, ice particles trailing from its wings, high fantasy digital art" --diffusion-fa -v --steps 4
+.\bin\Release\sd-cli.exe --diffusion-model ..\models\diffusion_models\lens_turbo_bf16.safetensors --llm "..\models\text_encoders\gpt-oss-20b-UD-Q8_K_XL.gguf" --tokenizer ..\models\tokenizers\tokenizer_gpt_oss.json --vae ..\models\vae\flux2_ae.safetensors --cfg-scale 1.0  -p "A crystal dragon soaring through an aurora borealis sky, its entire body made of transparent faceted crystal refracting the green and purple aurora light into rainbow spectra, ice particles trailing from its wings, high fantasy digital art" --diffusion-fa -v --steps 4
 ```
 
 <img width="256" alt="Lens Turbo example" src="../assets/lens/turbo_example.png" />

docs/pid.md

@@ -11,6 +11,8 @@ In stable-diffusion.cpp, PiD currently runs as an image edit pipeline: provide a
     - safetensors: https://huggingface.co/Comfy-Org/PixelDiT/tree/main/diffusion_models
 - Download Gemma 2 2B
     - safetensors: https://huggingface.co/Comfy-Org/PixelDiT/tree/main/text_encoders
+- Download Gemma 2 2B tokenizer.json
+    - https://huggingface.co/google/gemma-2-2b/tree/main
 - Download the VAE that matches the PiD checkpoint backbone
     - safetensors: https://huggingface.co/nvidia/PiD/tree/main/checkpoints
     - Flux / Z-Image PiD: use the Flux VAE and pass `--vae-format flux`
@@ -20,10 +22,12 @@ In stable-diffusion.cpp, PiD currently runs as an image edit pipeline: provide a
 
 The official PiD model card should be checked before use. At the time of the initial PiD release, the official weights are under the NSCLv1 non-commercial license.
 
+PiD and PiD 1.5 require an external Gemma 2 `tokenizer.json` matching the text encoder checkpoint. Save it as `tokenizer_gemma2.json` and pass it with `--tokenizer`; the tokenizer is not embedded in sd.cpp. See [JSON tokenizers](tokenizers.md) for CLI and C API usage.
+
 ## Examples
 
 ```
-.\bin\Release\sd-cli.exe --diffusion-model ..\models\diffusion_models\pid_flux1_512_to_2048_4step_bf16.safetensors --llm "..\models\text_encoders\gemma_2_2b_it_elm_bf16.safetensors" --vae ..\models\vae\ae.sft --vae-format flux --cfg-scale 1.0  -p "a lovely cat" -r ..\assets\ernie_image\turbo_example.png --diffusion-fa -v --steps 4 -H 2048 -W 2048 --rng cpu
+.\bin\Release\sd-cli.exe --diffusion-model ..\models\diffusion_models\pid_flux1_512_to_2048_4step_bf16.safetensors --llm "..\models\text_encoders\gemma_2_2b_it_elm_bf16.safetensors" --tokenizer ..\models\tokenizers\tokenizer_gemma2.json --vae ..\models\vae\ae.sft --vae-format flux --cfg-scale 1.0  -p "a lovely cat" -r ..\assets\ernie_image\turbo_example.png --diffusion-fa -v --steps 4 -H 2048 -W 2048 --rng cpu
 ```
 
 Before:

docs/tokenizers.md

@@ -1,8 +1,10 @@
 # JSON tokenizers
 
 Use a Hugging Face `tokenizer.json` to supply the tokenizer vocabulary, merges,
-added tokens, and processing stages. Without this option, sd.cpp keeps its
-embedded tokenizer for the selected model.
+added tokens, and processing stages. **PiD (including PiD 1.5) and Lens (including
+Lens Turbo) require an external JSON**; their Gemma 2 and GPT-OSS tokenizers are
+not embedded. Initialization fails if the main tokenizer is missing. Other
+models keep their embedded tokenizer when this option is omitted.
 
 ```shell
 sd-cli --diffusion-model model.gguf --llm text_encoder.gguf \
@@ -14,6 +16,13 @@ the embedding table does not establish that two vocabularies have the same
 meaning. The JSON file is loaded when the text encoder is created; its embedded
 vocabulary is not loaded in this case.
 
+| Model | Required text encoder tokenizer | Example |
+| --- | --- | --- |
+| PiD / PiD 1.5 | Gemma 2 matching the text encoder checkpoint | `--tokenizer tokenizer_gemma2.json` |
+| Lens / Lens Turbo | GPT-OSS matching the text encoder checkpoint | `--tokenizer tokenizer_gpt_oss.json` |
+
+The Gemma 3/4 tokenizer used by LTX-2 remains embedded.
+
 | Option | Encoder |
 | --- | --- |
 | `--tokenizer FILE` | Main LLM/BPE encoder: Gemma 2, Gemma 3, Qwen 2/3, Mistral, GPT-OSS; also Anima and HiDream-O1 |
@@ -44,7 +53,8 @@ sd-cli --diffusion-model sd3.gguf --clip_l clip_l.safetensors \
 ```
 
 The C API accepts the same string in `sd_ctx_params_t::tokenizer`. A null or
-empty value keeps the embedded tokenizers. The CLI passes the string through;
+empty value keeps an embedded tokenizer where available; PiD and Lens require
+a nonempty main tokenizer path. The CLI passes the string through;
 `TokenizerConfig` parses and validates it when text encoders are initialized.
 
 ```c