[Stage 1.5] Extract the private scheduler package and prove inlining

[draedful]

Parent: #318

Goal

Prove the private-package delivery model with a real internal package while keeping scheduler concepts available through the public graph contract.

The scheduler package exists only to enforce ownership and dependency direction. It must not become a published runtime dependency.

Scope

• Move the scheduler implementation into a private workspace package under packages/scheduler.
• Mark the scheduler package private and exclude it from publication.
• Make packages/graph consume the scheduler implementation through an explicit workspace dependency.
• Keep ESchedulerPriority and the supported schedule, debounce, and throttle contracts exported from @gravity-ui/graph.
• Keep Storybook, E2E, and Page Objects dependent only on @gravity-ui/graph.
• Configure the production build to inline the private scheduler implementation into the public graph artifact.
• Inspect the packed manifest and generated JavaScript for unresolved private-package imports.

Acceptance criteria

• packages/scheduler is private and cannot be published by the release workflow.
• @gravity-ui/graph remains the public owner and export surface for ESchedulerPriority.
• Page Objects import scheduler concepts only from @gravity-ui/graph.
• The @gravity-ui/graph tarball has no published dependency on the private scheduler package.
• Generated graph artifacts contain no unresolved import of the private scheduler package.
• Isolated vanilla JavaScript and React consumers pass without workspace access.
• Unit, Storybook, and E2E checks remain green.