[BUG]: HungarianMatcher.calculate_metrics reports a matched pair as both matched and fn/fp

[sromoam]

Problem

HungarianMatcher.calculate_metrics derives fn and fp as len(list) - tp, so a pair that appears in matched_pairs but scored below match_threshold is counted in both places at once:

m = HungarianMatcher(comparator=LevenshteinComparator(), match_threshold=0.9)
m.calculate_metrics(["abc", "def"], ["abd", "deg"])
# matched_pairs = [(0,0,0.667), (1,1,0.667)]   -> every item has a partner
# fn = 2                                        -> ...and yet 2 false negatives

This contradicts the documented model, where only items with no partner become FN or FA (Hungarian matching). HungarianMatcher is public API, exported from stickler.algorithms, so a caller reading fn as "GT items with no partner" gets the wrong answer at every list length.

Why it is not already fixed

StructuredListComparator reads matched_pairs and reclassifies, so no user-visible confusion-matrix metric is wrong today. The five-category split (TP/TN/FN/FA/FD) exists downstream; calculate_metrics predates it and only has the three-way vocabulary.

Documented as a Note on the method in #225 rather than fixed, because changing the return shape is not in scope for a fast-path bug fix.

Proposed direction

Give calculate_metrics an explicit fd key and make fn/fp mean what the docs say:

• fd: matched pairs scoring below match_threshold
• fn: GT items with no partner
• fp: fa + fd, preserving the existing identity

ComparisonHelper.unordered_list_metrics already produces exactly this shape, so the vocabulary and precedent exist.

Compatibility

Breaking for anything reading fn/fp off calculate_metrics directly. In-repo callers would need auditing; tests/common/algorithms/test_hungarian.py asserts these keys heavily.

Note tests/common/algorithms/test_hungarian_path_parity.py::test_zero_similarity_counts_are_length_independent pins today's fp == n / fn == n and carries a comment saying those values are descriptive rather than contractual. It will need updating alongside.

Acceptance criteria

• calculate_metrics returns fd, and fn/fp follow the documented partnering semantics
• The docstring Note added in fix(hungarian): align the single-item fast path with the general path (#224) #225 is replaced by the real contract
• Parity between the single-item shortcut and the general path is preserved (or the shortcut is removed first, see the fast-path deletion issue)
• CHANGELOG entry marked breaking for direct HungarianMatcher callers

Notes

Found by @vawsgit reviewing #225. Deferred from that PR deliberately.