fix(exporters): scope fp16 external-data cleanup to this model (data-loss)

[rylinjames]

Audit §3.3 / Part 1 #12 — a real data-loss bug.

Problem

convert_fp32_to_fp16's pre-save cleanup globbed every *.bin and *.data in dst.parent and unlinked them:

for pat in ("*.bin", "*.data"):
    for old in dst.parent.glob(pat):
        old.unlink()

The intent was only to clear leftover external data from a prior run of this model. But converting into a directory that also holds another model's weights (a shared export dir, an unrelated model.onnx.data) destroys those files.

Fix

Extracted _remove_stale_external_data(dst) — deletes only .bin/.data files whose name starts with dst.stem, never dst itself. Pure pathlib, unit-testable without onnx/torch.

Tests

tests/test_fp16_convert_external_data.py (5 cases, all pass):

• removes this model's stale .bin
• removes legacy .data / .onnx.data variants for this model
• preserves sibling models' weights (the data-loss guard)
• never unlinks dst itself
• no-op when the dir is clean

ruff clean on touched files.

🤖 Generated with Claude Code

[rylinjames]

I reproduced the remaining prefix-collision deletion (model2.bin and model_backup.data) and updated this branch. Cleanup now uses an exact allowlist for the current model.bin, legacy model.data, and legacy model.onnx.data paths; it no longer uses a prefix glob. The new regression preserves model2.bin, model_backup.data, and model-old.bin. Focused result: 6 passed; Ruff and diff checks pass.