Slim fixture: proto-umt5-small-slim

A 16.6M-parameter fixture for the T5 MIC, covering google/umt5-small. It is a test artifact, not a usable model, tagged test-artifact to keep it out of candidate-model search.

• Fixture key d2f1e2c7f664. It covers every checkpoint whose processor files are byte-identical and whose config matches after shrinking.
• Same architecture, smaller config: UMT5ForConditionalGeneration, shrunk by the generic rule in sweep.py (num_layers 2, num_decoder_layers 2).
• Processor files copied byte for byte from google/umt5-small: generation_config.json, special_tokens_map.json, spiece.model, tokenizer.json, tokenizer_config.json.
• Random weights (seed 17). They passed the input-sensitivity gate: every perturbation of the contract's inputs moved the logits by at least 0.001 (weakest: attention_mask:all_ones, 3.09), so no training was needed.
• Same tensor names as the real checkpoint. The real checkpoint has every parameter, and so does the fixture.
• Finite in float32, bfloat16, and float16 (forward, loss, gradients).

Built by build_fixtures.py; build.json records the pinned sources, gate results, and file hashes. The source repository commits only README.md and build.json; rebuild the other files with python example/build_fixtures.py --fixture proto-umt5-small-slim.