fix(player-core): keep video zoom when the next step is zoomed

[AbianS]

Problem

A zoomed video step always eased out to full frame shortly before its end. When the next step was also zoomed, the camera bounced:

• video → photo: the clip zoomed out, then the photo appeared at full frame and zoomed back in.
• video → video: the clip zoomed out, then the next clip zoomed back in. Most visible on contiguous clips, where playback rolls straight through the cut.

Photo → photo already moved straight from one zoom to the next.

Fix

• core/video-step.ts: new handsZoomTo(next). A zoomed clip keeps its zoom to the end when the next step (photo or video) has its own zoom. startVideoStep now takes the next step instead of a continuesIntoNext boolean and derives both decisions from it.
• Player.svelte: a zoomed photo that follows a zoomed video starts at the video's framing and moves to its own zoom with applyInheritedZoom, instead of resetting to full frame first. Video → video needs no change: the video layer stays visible and keeps its transform.
• When the next step has no zoom, the clip still eases out before its end, as before.

Tests

• video-step.test.ts: no zoom-out before a zoomed photo or a zoomed clip, zoom-out still happens before an unzoomed photo.
• Player.test.ts: from a zoomed video to a zoomed photo, and between two zoomed photos, the photo never passes through scale(1).
• The new video tests fail without the fix. The full player-core suite passes (118 tests), along with check-types, Biome and size (29.14 KB / 35 KB).