fix(core): picker labels honour data-label, then an image's alt

[vanceingalls]

What

The element picker's label for a picked element now uses the element's own data-label first, and an <img>'s alt text before falling back to "Image".

Why

The desktop app's new Cut out / Remove from picture feature writes cutout layers named in data-label (e.g. "Player cutout") and pictures with alt text. Picking one of them in Studio or the desktop preview showed a bare "Image", while the timeline already shows the author's name. The picker and the timeline now agree.

Related work

• Consumer: the desktop Cut out / Remove from picture stack in hyperframes-internal (packages/desktop). Desktop works without this change; after it merges, the desktop's vendored @hyperframes/core pin gets bumped so picks read the cutout's name.
• Stacked PR on top: docs(skills): hyperframes-core keeps data-hf-id and the derived-media lineage #4728.

How

buildElementLabel reads data-label (whitespace-collapsed, trimmed to the existing 56-character cap) before any tag-based rule; for img it uses alt the same way, then "Image".

Test plan

• Unit tests added/updated: packages/core/src/runtime/picker.test.ts (data-label wins for any tag, alt names an image, blank values fall back, long values are trimmed) — 47/47 pass
• Manual testing performed
• Documentation updated (if applicable)
• Comments follow CONTRIBUTING.md "Comments"

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