fix: start tour when async steps load

[wanxiankai]

Summary

• start the tour when run is already true and asynchronously loaded steps transition the store out of idle
• use the normal controls.start() path so the store reaches running, emits tour:start, and respects stepIndex / initialStepIndex
• add regression coverage for empty steps at mount followed by populated steps

Testing

• pnpm test (479 tests)
• pnpm typecheck
• pnpm exec eslint src test e2e (0 errors)
• pnpm build
• pnpm size
• pnpm typevalidation

Closes #1211

[codesandbox]

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