chore: hoist shared snapshot/selector test fixtures into a single canonical location

[thymikee]

Purpose

PR #2397 (selector pipeline move into @agent-device/selectors) introduced a second copy of shared test fixtures, forced by the layering rule that packages cannot import root src/:

• packages/selectors/src/internal/__tests__/snapshot-builders.ts mirrors root src/__tests__/test-utils/snapshot-builders.ts (duplicated makeSnapshotState; root copy keeps buildNodes for remaining root tests).
• packages/selectors/src/internal/__tests__/property-arbitraries.ts holds selector-chain arbitraries beside root src/__tests__/test-utils/property-arbitraries.ts (replay/gesture arbitraries).

Required behavior

• Shared test fixtures (snapshot builders, selector arbitraries) live in one canonical location; both root tests and packages/selectors tests import from it.
• Options: (a) a new test-fixtures subpath on @agent-device/selectors that root re-imports, or (b) a shared test-utils package. Choose whichever fits the existing layering model (root may import packages; packages may not import root src/).

Observable completion

• Exactly one copy of each fixture helper exists in the repo.
• Root and package tests import it through the chosen subpath/package.
• pnpm check:affected --run passes.

Dependencies

• PR refactor(move): move the selector pipeline and interaction targeting into @agent-device/selectors #2397 must be merged first (fixtures land in the selectors package there).
• Check with scripts/layering/package-boundaries.test.ts pins and the contracts exports snapshot when adding a new subpath.