Dev4: ONNX export pipeline for Option A deployment

[sp25-bai-047-wq]

Summary

Implements the ONNX export path (Option A) for solving the Vercel serverless size limit (~250MB) that currently prevents the neural model from running in production.

What's included

• inference/grammar.py — torch-free grammar/vocab helpers (is_valid_prefix, NodeValidityPool, vocab loading), split out of beam_search.py so the ONNX deployment path never needs to import torch.
• inference/beam_search.py — updated to import grammar helpers from the new grammar.py (no logic changes).
• deployment/export_onnx.py — exports the trained SimpleCalculusModel checkpoint (model/simple_transformer.py) to ONNX format.
• deployment/onnx_beam_search.py — numpy/onnxruntime-only mirror of inference/beam_search.py's beam search.
• deployment/onnx_solve.py — torch-free mirror of inference/solve.py's solve() interface, for use in production.
• deployment/verify_export.py — runs eval/benchmarks/*.json through both the PyTorch and ONNX paths, checks exact-match output parity, measures real deployment bundle size against Vercel's 250MB limit, and writes docs/EXPORT_DECISION.md from the actual measured results.
• requirements-onnx.txt — lightweight production dependencies (onnxruntime + numpy), deliberately excludes torch.
• api/_shared.py — added a new priority step in get_solver(): local ONNX model is now tried before the heavier local torch load, falling back to Groq/FallbackSolver as before if unavailable.

Status

Export and verification scripts are ready and tested against the current architecture (model/simple_transformer.py's SimpleCalculusModel). Final numbers in docs/EXPORT_DECISION.md should be regenerated once run against the latest signed-off checkpoint from Developer 3, per the re-validation rule in the task assignment doc.

Testing

• All new files pass py_compile with no syntax errors.
• inference/grammar.py functionally tested against the real tokenizer/vocab.json (is_valid_prefix, load_vocab).
• Existing beam_search.py logic unchanged — only the grammar helpers were relocated.

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