Add color picker gradient color stop duplication by holding Alt

[Keavon]

No description provided.

[gemini-code-assist]

Code Review

This pull request implements Alt-drag duplication of gradient stops, allowing users to clone a marker by holding the Alt key during a drag. It adds new message variants (InsertDuplicate and RemoveDuplicate), updates the Svelte frontend to track Alt key states and reconcile duplicates, and introduces a duplicate_stop method in the Rust backend. The review feedback highlights three key issues: a race condition where calling reconcileDuplicate immediately on key events bypasses safety guards and causes the wrong marker to move, a potential bug in findDuplicateAnchorIndex that could delete unrelated markers if the duplicate is missing (suggesting a distance threshold), and a missing defensive guard in the Rust backend to prevent deleting the active marker itself during duplicate removal.

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[cubic-dev-ai]

4 issues found across 4 files

Confidence score: 2/5

• In frontend/src/components/widgets/inputs/SpectrumInput.svelte, the duplicate-release path can remove the nearest stop even when it doesn’t match the frozen duplicate position, so stale marker props could delete the wrong gradient stop for users — tighten the removal check to only act on an exact frozen-position match before merging.
• In frontend/src/components/widgets/inputs/SpectrumInput.svelte (reconcileDuplicate, moveActiveMarker, and the Alt keydown flow), reconciling duplication too early creates a race that bypasses the stale-index guard and can move an unintended stop after mid-drag duplication — route reconciliation through the guarded path and verify Alt down/up drag behavior with a targeted regression test before merging.
• In editor/src/messages/layout/utility_types/widgets/input_widgets.rs, the un-duplicate doc wording is a minor clarity issue and not merge-blocking, but updating it to standard phrasing will prevent confusion in future maintenance.

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[cubic-dev-ai]

1 issue found across 3 files (changes from recent commits).

<sub>Reply with feedback, questions, or to request a fix.

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