fix(parsers): preserve authored keyframe intent

[miguel-heygen]

What

Preserve authored keyframe intent through GSAP parsing, mutation, serialization, and Acorn writer parity.

This is Family A, PR 2 of 4. Prerequisite: codex/studio-timeline-a-runtime-eases-v2. Next: codex/studio-timeline-a-ease-editor-v2.

Replacement review history: #2582. Golden reference: #2387.

Why

Keyframe editing must not silently collapse duplicate-time data, discard unrelated properties, change authored source form, or inject timing that was never authored.

How

Parser and writer paths share the existing easing and serialization contracts, retain distinct authored entries, and merge edits into the targeted keyframe. Two small helper extractions keep the mutation owner explicit while satisfying the repository complexity gate.

Test plan

• Unit tests added/updated — 429 focused parser tests passed with 5 intentional skips at this PR boundary
• Manual testing performed — parser/writer behavior is included in the certified integrated QA matrix
• Documentation updated (not applicable; internal parser behavior)

[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.
Learn more

• fix(parsers): preserve duration-authored keyframe timing #2675
• feat(studio): add keyframe ease editor #2674
• fix(parsers): preserve authored keyframe intent #2673 👈 (View in Graphite)
• feat(core): add deterministic keyframe ease runtime #2672
• main

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