Lift `apply_viewport`'s three-way diff (resize / set_hidpi / set_zoom)
into a `ViewportChange` value so the decision is testable without a
running `SoftwareServoHost`. Four unit cases pin the contract:
- Retina creation (DPR=2.0): only the hidpi push fires (the bug above).
- Standard-DPI creation (DPR=1.0): nothing to push, session already
matches Servo's defaults.
- Mid-session zoom change: only `set_page_zoom` fires.
- Cross-monitor DPR change: only the hidpi push fires.
This is a regression guard for the previous commit — any future change
that re-initializes a session from request values instead of Servo's
post-build defaults will fail `fresh_retina_session_pushes_hidpi_only`.
ZacharyZhang-NY
4cbd1c8870