fix: derive spring duration from physics parameters

[Iams4kura]

Summary

• derive the perceived duration and bounce when stiffness or damping is supplied to the Spring constructor
• preserve the existing precedence of explicit bounce / duration parameters and the default behavior for unrelated parameters
• cover the reported callback threshold with a browser regression test

Fixes #1118

Regression evidence

• Before: the new regression failed with expected 628 to equal 1147.147.
• After: the same regression passes and verifies that onComplete does not fire at 628ms, then fires at the derived 1147.147ms threshold.

Verification

• Eases browser suite: 52 passing
• Full browser suite: 599 passing
• npm run build
• git diff --check

Scope

• 2 files changed, +18 / -0 lines; no generated files or dependency changes