feat: dynamically layout bilingual ASS subtitles

[Mapleeeeeeeeeee]

Summary

• Add dynamic bilingual ASS layout so translated and original subtitle lines stay grouped
• Wrap source/translated text with top-anchored ASS positions to avoid multiline English pushing upward
• Add unit coverage for 1, 2, and 3-line original subtitle cases plus burn-in integration coverage

Verification

• uv run ruff check src/ tests/
• uv run ruff format --check src/ tests/
• uv run mypy src/
• uv run pytest tests/unit -m unit --cov=bilingualsub --cov-report=xml --cov-fail-under=60
• uv run pytest tests/integration -m integration --cov=bilingualsub --cov-report=xml --cov-fail-under=30
• uv run pytest tests/e2e -m e2e --cov-fail-under=0