Fix dotted abbreviation sentence boundaries

[vanderlee]

What changed

• Treat the doubled final period in dotted-initial abbreviations such as a.m.. and A.M.. as a sentence boundary.
• Prevent fragments ending in multiple periods from being reclassified as abbreviations.
• Preserve the existing handling of ordinary ellipses and short abbreviations.

Why

The current parser recognizes a. as an abbreviation, but punctuation merging leaves m.. How about Greg? in one fragment. Abbreviation merging then joins both sentences. For uppercase initials, M.. is also incorrectly classified as an abbreviation because it starts with a capital letter.

This makes the existing testSplitLowercaseInitialAbbreviations regression fail in every PHP matrix job.

Impact

Dotted abbreviations can now end a sentence without swallowing the following sentence. Existing abbreviation and ellipsis behavior remains covered by the test suite.

Validation

• PHPUnit 9.6.36 on PHP 8.5.9: 36 tests, 122 assertions
• PHP syntax check for src/Sentence.php
• Focused checks for lowercase and uppercase a.m.. boundaries, e.g.., ordinary ellipses, and existing double-period behavior