fix(player-core): carry photo zoom into a zoomed video step

[AbianS]

Closes #3

Problem

A video step only inherited the previous photo's zoom when it had no panZoom of its own. With its own zoom, it ran the regular entry animation: an instant reset to full frame, then a zoom in.

Fix

• applyInheritedZoomOut becomes applyInheritedZoom(target, inherited, own, …). The video starts instantly at the photo's framing and, on the next frame, moves:
  • to its own zoom with its own duration/easing, like two consecutive zoomed photos, or
  • back to identity with the photo's timing when it has no zoom (unchanged behavior).
• Player now inherits the previous photo's zoom for every video step.

Tests

• Unit test in media-zoom.test.ts: zoom to zoom, never through identity.
• Integration test in Player.test.ts: from a zoomed photo to a zoomed video, the video layer's first transform is the photo's zoom, it never passes through scale(1), and it settles on the video's zoom. It fails without the fix.
• Full player-core suite passes (113 tests), along with check-types, Biome and size (29.07 KB / 35 KB).