Explain empty interfaces.csv instead of writing it silently (#17)

[DimaMolod]

Problem

Fixes #17. For a small fraction of AF3 heterodimer predictions, process() produced a zero-byte interfaces.csv (no header) while the log still said wrote .../interfaces.csv with no error. A genuine "no interface" result was indistinguishable from a parse/load failure.

Root cause

AlphaJudge only emits interface rows for chain pairs with at least one inter-chain contact within --contact_thresh (default 8 Å, Cβ–Cβ). When AlphaFold places the two chains with no contact at that cutoff, comp.interfaces is empty, rows stays empty, and the old code wrote an empty file silently:

else:
    f.write("")
logger.info(f"wrote {out}")

This is reproducible on re-prediction because those models genuinely have non-contacting chains.

Fix

• Track lightweight per-run diagnostics (models processed, interfaces detected, interfaces dropped by --pae_filter).
• When rows is empty, emit a WARNING explaining why the CSV is empty and which knob to relax (e.g. no inter-chain contacts within contact_thresh=8.0 A ... try a larger --contact_thresh).
• File-write behavior is unchanged (still an empty file) for backward compatibility — only the logging improves.

Test

Adds a regression test that runs the AF3 fixture with a sub-Ångström contact_thresh, asserting the empty CSV plus the explanatory warning. Targeted runner tests pass (4 passed).

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