Add generator-neutral property testing for XState v6

[davidkpiano]

Summary

• add generator-neutral propertyTest() to xstate/graph, accepting machines and test models
• add exact state-node, configuration, event, transition, guard, and frontier coverage with covered/uncovered/unreachable/unknown classifications
• add frontier selection and per-frontier budgets while preserving deterministic prefixes during shrinking
• add independent reference-oracle and optional SUT equivalence seams
• add chronological event/clock/checkpoint/stop commands, temporal properties, portable replay fixtures, and readable traces
• add the optional @xstate/fast-check adapter plus an Effect Schema entrypoint without coupling XState core to either dependency

Why

Existing shortest/simple path and model-based testing cover finite representative events. This adds varied payload and sequence exploration, shrinking, deterministic replay, continuous invariants, exact transition evidence, temporal checks, and runtime/reference equivalence while preserving pure XState transition calculation.

Validation

• pnpm test — 2,681 passed, 34 skipped, 3 todo
• pnpm typecheck
• pnpm lint
• targeted formatting check for every changed source, test, package, README, workspace, and changeset file

pnpm format:check still reports only the unchanged baseline packages/core/CHANGELOG.md.

[changeset-bot]

🦋 Changeset detected

Latest commit: a5b0f21

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
xstate	Minor
@xstate/fast-check	Minor

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