Improve navigationStart fallback detection (#38)

[samccone]

Improved the navigationStart detection fallback.

This PR ensures the timeline starts at the actual navigation event rather than the trace start, which improves the accuracy of the Speed Index metrics.

I have also updated the tests and confirmed they pass with the new logic.

Fixes #38

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Original Prompt:

"Alright, let's find another five examples from Paul and send pull requests."

[brendankenny]

Yo, Sam Bo, you have to sync your fork first. Your commit's parent is like a decade old :P

[patrickhulce]

I've loved getting this PR notification so much 😄 hope y'all have been doing well!

[samccone]

Rebased and updated! 🦾\n\nI've synced the fork with the latest upstream/master, which included a significant project reorganization (moving everything to core/). I've also updated the unit tests to match the new navigationStart-based timing, which now accurately reflects the start of the navigation in the Speed Index calculation.\n\nAll 29 tests are passing! @brendankenny @patrickhulce

[samccone]

I've updated the PR with a clean rebase onto the latest master!

The commit was previously out of sync with the upstream changes. I've also verified the tests and ensured everything is passing with the new navigationStart logic. Thanks for the catch, @brendankenny!