Add optional multi-page HAR generation

[tinyopsstudio]

Refs #65.

Summary:

• add an opt-in allowMultiPage option while preserving the current one-page default
• create a new HAR page for root-frame scheduled/document navigations when the option is enabled
• map responses through the entry pageref so second-page document and subresource timings use the correct page
• cover default compatibility plus the opt-in multi-page navigation behavior with a regression test

Verification:

• npm test -> 30 tests passed
• npm run lint -> passed