feat(ui-core): add worker timeline layout helpers

[andrewkcchung]

Why

Timeline renderers need the same deterministic answers for visible-span zoom and worker-lane order regardless of which product surface renders them. Keeping these calculations as pure UI-core helpers prevents renderer-specific copies, makes layout behavior testable without a DOM, and provides a reusable foundation for current and future timelines.

What changed

• Add a pure helper that derives bounded timeline geometry and zoom from the visible time span.
• Add deterministic lane ordering that preserves known choices and appends newly observed lanes stably.
• Export both helpers from the shared UI core.
• Cover boundary snapping, midpoint behavior, geometry caps, duplicate lanes, and input immutability.

Behavioral proof

Command:

node --test packages/app/ui/core/test/worker-timeline-layout.test.mjs

• Red — test-only patch on main: The test failed because the shared timeline helper modules did not exist on main.
• Green — this branch: 5 tests passed.

Risk assessment

Relative risk: Low

The contribution is additive, pure, DOM-independent UI-core logic and does not yet replace an existing renderer path. Its risk is limited to the public geometry and ordering contract exposed for future consumers. Deterministic boundary, duplicate-lane, immutability, and chart-cap tests constrain that behavior.

Review focus

• Confirm the fixed lane, gutter, row-height, and maximum-chart dimensions are suitable shared UI-core contracts.
• Review zoom snapping at exact level boundaries and midpoint selection between levels.
• Confirm duplicate and newly discovered lane IDs retain deterministic ordering.
• Confirm the helpers remain renderer- and DOM-independent.