perf(duration): avoid transient bound functions

[OskarEichler]

Summary

• delegate ordinary add and subtract calls with Function#call
• avoid constructing and immediately invoking a bound function on each non-duration operation

Behavior

This preserves the receiver, arguments, duration-specific branches, return values, and public APIs. It is a focused implementation simplification; no breaking change is intended.

The existing duration suite already covers both ordinary numeric operations and duration operands, so no assertion-only regression was added for this behavior-preserving change.

Verification

• focused duration suite: 30 tests passed
• npm test: 93 suites, 794 tests passed
• npm run lint
• NODE_OPTIONS=--openssl-legacy-provider npm run build
• package size remains within the 2.99 KB limit
• git diff --check