Document the export-format support matrix per architecture and tier

[maziyarpanahi]

Summary

Export support is fragmented across openmed/mlx (bert/deberta/privacy-filter/gliner families, INT8/INT4), openmed/coreml (float16/32, token-classification only), and the new ONNX/WebGPU/Transformers.js/GGUF paths, with real limitations (GGUF embeddings only; CoreML token-classification only; MLX family allowlist). There is no single doc telling contributors which (architecture x format x tier) combinations are supported, so users guess and file issues. This is a self-contained docs task with a coherence test, ideal for a first contribution.

Scope

• Author docs/export-matrix.md as a table of architecture-family rows (BERT/DistilBERT/Electra/RoBERTa/XLM-R/DeBERTa-v2/ModernBERT/Longformer/privacy-filter/GLiNER) by export format columns (MLX-fp, MLX-8bit, MLX-4bit, CoreML-fp16, CoreML-int8, ONNX, Transformers.js, GGUF), marking supported / partial / unsupported with the reason.
• Capture the known limitations verbatim: GGUF is embedding-only, CoreML is token-classification-only, the MLX family allowlist, INT4 only-if-recall-holds.
• Add a small test that parses export-matrix.md and asserts every family in the MLX _SUPPORTED_TOKEN_CLASSIFICATION_MODEL_TYPES allowlist has a row (keeps the doc honest as backends are added).
• Link the matrix from the existing export docs.

Acceptance criteria

• docs/export-matrix.md exists with an architecture-by-format support table and per-cell reasons for partial/unsupported.
• A test asserts every MLX-supported family appears as a row in the matrix.
• The matrix captures the GGUF-embedding-only and CoreML-token-classification-only limitations.
• test suite green: .venv/bin/python -m pytest tests/ -q

Out of scope

• Implementing any new export backend.
• Auto-generating the matrix from code (a static doc + coherence test is sufficient).

Files

• docs/export-matrix.md
• tests/unit/test_export_matrix.py

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Task: OM-146 · Milestone: Backlog · Priority: P3 · Size: S
Depends on: — · Blocks: —
Roadmap: section 2.2c; Appendix C (export file index)
Spec: PLANS/V2/EXECUTION/tasks/OM-146.md