Inconsistent entry.time across browsers

Spaniel's polyfill for `IntersectionObserver` uses `Date.now()` for setting `entry.time`, which is relative to epoch. The native implementation defines `entry.time` as being relative to the creation of the document (ref: https://developer.mozilla.org/en-US/docs/Web/API/IntersectionObserverEntry/time).

Start by locating Spaniel's IntersectionObserver polyfill and the code that assigns entry.time with Date.now(). Compare that value with the document-relative timing required by the native API. Done means the polyfill's entry.time matches native semantics across browsers; the issue names no specific file or test to run.

Written by the indexing model from the issue text.