fix: ignore tiny secondary subtitle overlaps

[kuator]

Problem

Mpvacious includes every secondary subtitle that overlaps the selected primary
subtitle, even when the overlap is extremely small. This can add an unrelated
line from just before or after the selected dialogue.

Example: tiny overlap

Suppose the selected primary subtitle starts at 10.0 seconds, while the
previous secondary subtitle ends at 10.1 seconds. They overlap by only 0.1
seconds, but mpvacious can still include that previous subtitle in the card.

This change ignores such tiny overlaps. An overlap must cover at least half of
the shorter subtitle. For very short subtitles, it must cover at least 75%.

Example: tracks split dialogue differently

The primary track may split one sentence into two subtitle lines while the
secondary track keeps it as one line, or the other way around. Checking only
the full selected time range can then miss the matching translation.

This change checks both the complete selected range and each primary subtitle
inside it. That lets mpvacious find the matching secondary text without also
accepting barely overlapping neighbouring lines.

Change

• Ignore secondary subtitles whose overlap is too small to be meaningful.
• Use a stricter threshold for subtitles shorter than one second.
• Check individual primary subtitles as well as the full selected range.
• Return the matching secondary subtitles for the preview.

Tests

Added tests for tiny boundary overlaps, very short subtitles, normal partial
overlaps, and tracks that split the same dialogue differently. The full test
suite passes with luajit tests/run.lua.

[kuator]

#177

[tatsumoto-ren]

Thanks!