server : accept images on /decision, add harness example

[graydini]

server : accept images on /decision, add harness example

What

POST /decision gains an optional images field. Each entry maps 1:1 to a
context and carries base64 image data, or an array of base64 strings when one
context should see several images. A data URL prefix is accepted and stripped.
The decoded bitmaps reach the decision engine through
options::context_bitmaps.

Media markers are prepended only when the context text does not already contain
them. A caller that interleaves markers with its own labels keeps control of
where each image lands; a caller that just wants "this image, this question"
keeps the old behaviour unchanged.

Engine

A prompt_part is now either a list of text tokens or a media chunk.
tokenize_mm expands media markers into chunks and reports text tokens with
LLAMA_TOKEN_NULL at the image positions. decide_batch turns that into
interleaved parts, and decode_parts encodes chunks through the mtmd batch API
(mtmd_batch_init / add_chunk / encode / get_output_embd) before the
surrounding text, which is the path the completion endpoint already uses.

Models that reject a second clip chunk with "batch too large" fall back to
encoding one chunk at a time. SmolVLM2 needs that fallback; Qwen2.5-VL takes
the batch path.

LLAMA_DECISION_DEBUG in the environment traces this path.

Example

examples/vision-decision-harness/ is a Flask UI that drives the endpoint over
images and text: folder upload, batch runs with SSE progress, image selection
across a folder, a text classification suite, and snippet export. It is an
example, not a dependency, and nothing in the server links against it.

Notes

• Vision encoding and branch scoring still complete in one batched pass per
  group. There is no autoregressive loop.
• Text-only decisions never enter the mtmd path. The batch is only constructed
  when a context actually has a chunk.
• Verified on Qwen2.5-VL-7B and SmolVLM2-500M. Multi-image selection over a
  12-image folder picks the intended image; the text suite runs 20 files at
  ~80% on the 7B.

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