fix(studio): scope timeline context targets

[miguel-heygen]

What

Scope timeline context-menu mutations to the exact clicked element, animation, property lane, and keyframe.

Why

Selection state can be stale or point at a different clip when a context action runs. Ambiguous fallback can remove or mutate the wrong lane, especially when several animations share one element and percentage.

How

• Route context actions through the shared resolveTimelineKeyframeTarget owner.
• Carry explicit animation and lane identity through keyframe, gap, and overlay context paths.
• Reject stale explicit identities instead of falling back to a different animation.
• Add regressions for non-selected elements, stale identities, and delete-all lane targeting.

This is E5 of the independent Family E draft Graphite stack, stacked only on E1-E4 and the immutable Family D review baseline.

Test plan

• Unit tests added/updated
• Manual testing performed
• Documentation updated (not applicable)

Validated on the exact Family E tip with 130 focused interaction tests, 3 timeline virtualization tests, the full Studio suite (2,991 passed; 18 todos), Studio typecheck/build, the full workspace build, oxfmt, oxlint, diff hygiene, and file-size/Fallow gates.

[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(studio): scope timeline ease focus lifecycle #2710
• perf(studio): stabilize virtualized beat gestures #2709
• fix(studio): scope timeline context targets #2708 👈 (View in Graphite)
• perf(studio): virtualize timeline marquee selection #2707
• perf(studio): stabilize virtualized timeline drops #2706
• perf(studio): stabilize virtualized keyframe retiming #2705
• perf(studio): stabilize virtualized clip gestures #2704
• codex/review-baseline/studio-timeline-family-d-v2

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