Fix spurious "Cannot determine terminus type" debug message for asymmetric paired hits

[Copilot]

In multi-pair --pairing-map runs, writeFlanks iterates over all models in hitIndex (which contains hits from every pair), but each call receives only the current pair's PairingConfig. For models belonging to other pairs, _determine_terminus_type correctly returns None — but this incorrectly triggered the misleading debug message and silently skipped hits that are classifiable, just in their own pair's context.

Changes

• src/tirmite/tirmitetools.py — In writeFlanks, precompute config_models (the set of left_model/right_model for the current asymmetric config) before the unpaired hits loop. Models not in this set are silently skipped; they will be processed with the correct config in their own pair's writeFlanks call.

# Only built for asymmetric configs; symmetric configs continue to use
# strand-based classification for all models as before.
config_models: Optional[Set[str]] = None
if config is not None and config.is_asymmetric:
    config_models = {m for m in (config.left_model, config.right_model) if m is not None}

if write_all:
    for model in hitIndex.keys():
        if config_models is not None and model not in config_models:
            continue  # foreign model — silently skip
        ...

• tests/test_flank_extraction.py — Added test_asymmetric_config_skips_foreign_models_silently: builds a hitIndex with models from two asymmetric pairs, calls writeFlanks with one pair's config, and asserts that (a) only that pair's models produce flank files and (b) no "Cannot determine terminus type" log messages are emitted for the foreign models.

[Adamtaranto]

@copilot fix failing tests