Fix year precision for duration and elapsed formats

[Pybsama]

Summary

• honor precision="year" when calculating elapsed durations
• preserve the existing empty-duration result for an unknown runtime precision
• cover positive and negative helper results plus rendered duration and elapsed output

Root cause

unitNames.indexOf("year") returns 0, but the logical-or expression treated
that valid index as falsy and replaced it with unitNames.length. Using the raw
index keeps 0 as the year cutoff and also preserves the existing -1
behavior for an unknown runtime value.

Testing

• npm test (634 passing)
• npm run lint
• npm run build

Fixes #366