fix(da): restore lowercase dotted abbreviation boundaries

[yisding]

Motivation

• A recent split-mode change only treated two-letter initialisms as split candidates when both letters were uppercase and removed the Danish-specific boundary-abbreviation override, causing lowercase/mixed-case Danish dotted abbreviations like s.u./s.U. to be under-segmented before capitalized followers.
• The change is a correctness regression for Danish sentence-splitting and should be fixed narrowly without changing global two-letter initialism heuristics.

Description

• Add a per-replacer opt-in SENTENCE_BOUNDARY_ABBREVIATIONS attribute to AbbreviationReplacer so languages can list multi-period abbreviations that should be treated as sentence-boundary candidates.
• In replace_multi_period_abbreviations() compute abbreviation_key = matched[:-1].lower() and use listed_sentence_boundary to influence the final-period restoration decision (treat listed entries as split candidates before likely sentence starts).
• Configure Danish AbbreviationReplacer to include SENTENCE_BOUNDARY_ABBREVIATIONS = frozenset({"s.u"}) so s.u. and s.U. split correctly before a capitalized follower.
• Add golden-rule tests for the Danish cases and update the regression snapshot to reflect the restored behavior.

Testing

• Ran targeted tests with uv run pytest tests/regression/test_segment_snapshot.py tests/unit/test_abbreviation_replacer.py tests/regression/test_greek_abbreviations.py tests/lang/test_danish.py and all collected tests passed.
• Updated the regression baseline with uv run python -m tests.regression.segment_snapshot --update and verified the snapshot-based regression test tests/regression/test_segment_snapshot.py passed after the update.
• Ran the full suite with coverage via uv run pytest --cov=sentencesplit tests/ --color yes and observed the test suite completed with the expected pass/skip/xfail summary and coverage threshold met.

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Codex Task

[charliecreates]

Reviewed the current diff and test updates, and I don’t have actionable code feedback to address. CI checks for this head commit are still running, so merge readiness still depends on those results.