fix(seed): seed train and eval randomness

[rylinjames]

Summary

• add shared seeding helpers for Python, NumPy, Torch, CUDA, and child-process PYTHONHASHSEED
• seed finetune runs before preflight/backend work and pin lerobot subprocess hash seeding
• forward LIBERO Modal eval seeds through modal run and seed Torch noise in the remote script

Tests

• PYTHONPATH=$PWD/src /Users/romirjain/Desktop/building\ projects/fastcrest/tether/.venv/bin/python -m pytest tests/test_finetune.py tests/test_eval_modal_runner.py tests/test_seeding.py -p no:cacheprovider
• PYTHONPATH=$PWD/src /Users/romirjain/Desktop/building\ projects/fastcrest/tether/.venv/bin/python -m pytest tests/test_finetune_preflight.py tests/test_distill_cli_integration.py tests/test_finetune_phase_a.py -p no:cacheprovider
• /Users/romirjain/Desktop/building\ projects/fastcrest/tether/.venv/bin/ruff check src/tether/seeding.py src/tether/finetune/run.py src/tether/eval/modal_runner.py scripts/modal_libero_monolithic_onnx.py tests/test_finetune.py tests/test_eval_modal_runner.py tests/test_seeding.py
• PYTHONPATH=$PWD/src /Users/romirjain/Desktop/building\ projects/fastcrest/tether/.venv/bin/python -m py_compile src/tether/seeding.py src/tether/finetune/run.py src/tether/eval/modal_runner.py scripts/modal_libero_monolithic_onnx.py

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