feat(studio): GSAP runtime bridge + optimistic updates [3/6]

[miguel-heygen]

Summary

Runtime bridge reads gsap.getProperty() from iframe for drag commits. Optimistic update pattern for keyframe mutations. Async pipeline with beforeReload/skipReload.

Part 3 of 6 — drag infrastructure. Depends on PR 2.

Test plan

• Drag GSAP element → position persists via GSAP mutation (not CSS offset)
• Optimistic cache updates roll back on failure

[miguel-heygen]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• feat(studio): design panel, timeline polish, feature flag [6/6] #1172
• feat(studio): keyframe hooks wiring — session, cache, toolbar [5/6] #1171
• feat(studio): keyframe diamonds, navigation, context menu [4/6] #1170
• feat(studio): GSAP runtime bridge + optimistic updates [3/6] #1169 👈 (View in Graphite)
• feat(core): spring physics solver + runtime fixes [2/6] #1168
• feat(core): GSAP keyframe parsing, mutations, and API routes [1/6] #1167
• main

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