fix(parsers): preserve duration-authored keyframe timing

[miguel-heygen]

What

Preserve duration-authored timing when parsing and editing GSAP object-array keyframes.

This is Family A, PR 4 of 4. Prerequisite: codex/studio-timeline-a-ease-editor-v2. It is the Family A tip.

Review context: #2582 and the post-certification review fixes for #2387.

Why

Object-array keyframes use per-step durations as their timing source. Treating them as evenly distributed percentages can create, update, or remove a keyframe at the wrong authored position.

How

A single timing helper owns duration-to-percentage conversion and index resolution. Parser and writer mutations use that shared result, preserve per-step durations, and synthesize an outer duration only when the source does not already provide one.

Test plan

• Unit tests added/updated — 434 focused parser/writer tests passed with 4 intentional skips at this PR boundary
• Manual testing performed — duration-authored add, update, remove, and round-trip behavior is covered by 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 👈 (View in Graphite)
• feat(studio): add keyframe ease editor #2674
• fix(parsers): preserve authored keyframe intent #2673
• feat(core): add deterministic keyframe ease runtime #2672
• main

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