Implement `object-fit` and `object-position` for replaced elements

[FlorianRappl]

Description
Replaced elements (especially <img>) appear to stretch to the element box instead of honoring object fitting and positioning rules.

Scope

• Implement object-fit modes (fill, contain, cover, none, scale-down).
• Implement object-position alignment/offset handling.
• Apply logic to replaced-content rendering pipeline with intrinsic dimensions.

Acceptance criteria

• Images render according to specified object-fit mode.
• object-position moves focal position as expected.
• Aspect-preserving and cropping behavior matches CSS expectations for test fixtures.