Recognize single-word period sentences

[vanderlee]

What changed

• Replace the broad “short capitalized token” abbreviation heuristic with explicit handling for common abbreviations, initials, dotted acronyms, Roman numerals, and numeric/list markers.
• Recognize ordinary one-word sentences ending in a single period without treating ellipses as boundaries.
• Remove empty fragments produced when trimmed line-separated list items are returned.
• Add regressions for Go. Stop. Wait., the example from issue "Sentences must be at least two words long, unless a linebreak or end-of-text." #4, ordered lists, ellipses, numeric sentence endings, and existing abbreviation behavior.
• Update the documented boundary rule.

Why

The previous heuristic could not distinguish Go. from an abbreviation because any short capitalized word ending in a period was treated as one. Ordered-list prefixes still need to remain attached to their item text, and legacy abbreviation and ellipsis behavior must remain stable.

Validation

The focused patch workflow passed all 35 tests and 123 assertions on PHP 8.4. The complete stacked GitHub Actions matrix also passed on PHP 5.4, 5.6, 7.4, 8.0, 8.4, and 8.5.

This broader abbreviation classification overlaps the narrower change in PR #33 and should replace that parser portion rather than be merged independently after it.

Closes #4.
Closes #23.

[vanderlee]

Currently explicitely support english hardcoded abreviations. Do not merge as is.