Support v1 raw multimodal image offload

[eligotts]

Summary

Wires Prime-RL v1 training onto the generic raw multimodal image offload contract used by the companion Verifiers and Renderers PRs.

Companion PRs:

• Renderers: [codex] Support raw image refs for multimodal rendering renderers#89
• Verifiers: [codex] Support raw image offload in v1 train client verifiers#1746

What changed:

• Adds a prime_rl.multimodal adapter layer and strict raw multimodal descriptor schema for adapter-owned payloads.
• Teaches /inference/v1/generate to consume renderer mmraw refs as generic RawMMRef objects, build RawMMItem from family/fingerprint/payload, and delegate materialization to the registered adapter.
• Converts v1 traces into trainer-bound mm_refs carrying strict prime_raw_mm_item envelopes plus local file:// image URIs; legacy eager mm_kwargs/processed payloads are rejected.
• Materializes raw image refs in the trainer with the trainer model's HF image processor and adapter-specific validation.
• Keeps multimodal samples compatible with packing by carrying raw refs and image token type ids through TrainingSample and packed MicroBatch.
• Keeps trainer.missing_mm_image_policy, defaulting to placeholder_zero_loss, with placeholder synthesis delegated to the owning multimodal adapter.
• Adds adapter coverage for Qwen-VL and Kimi K2.5, including Qwen processor SizeDict.get(...) handling.

Validation

• Pre-commit during commit: ruff check and ruff format passed.
• End-to-end hosted-style smoke using the intended local stack:
  OUT=/tmp/prime-rl-v1-raw-mm-offload-clean-api-smoke-20260620-072559 PYTHONPATH=/home/ubuntu/prime-rl-v1-raw-mm-offload/src:/home/ubuntu/renderers:/home/ubuntu/verifiers scripts/smoke/mini_browse_platform_v1_raw_offload.sh
• Smoke result: inference started, Prime env sandboxes ran, orchestrator collected 4/4 trainable mini-browse rollouts, trainer completed step 0, and decoded trainer-bound artifacts had 4 TrainingBatch examples and 4 packed MicroBatches with mm_refs, 0 eager mm_kwargs, 12 strict prime_raw_mm_item descriptors, 12 local file:// image URIs, 0 data URIs, 0 missing files, and image token type ids present.