fix: use globalThis for step registry to survive module duplication

[jcourson-bg]

Summary

• Store registeredSteps on globalThis via Symbol.for() so all module instances share a single registry, matching the existing pattern in world.ts

Problem

In Turbopack dev mode, private.js can be instantiated as separate module instances across different chunk contexts. registerStepFunction (called from the generated step route) and getStepFunction (called from the step handler) end up operating on different Map instances, causing "Step not found" errors.

Test plan

• pnpm build passes
• pnpm test in packages/core
• Manual: start Next.js dev server with Turbopack + postgres world, trigger a workflow with steps, confirm no "Step not found" errors

[changeset-bot]

🦋 Changeset detected

Latest commit: 91bb717

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

This PR includes changesets to release 16 packages

Name	Type
@workflow/core	Patch
@workflow/builders	Patch
@workflow/cli	Patch
@workflow/next	Patch
@workflow/nitro	Patch
@workflow/vitest	Patch
@workflow/web-shared	Patch
workflow	Patch
@workflow/world-testing	Patch
@workflow/astro	Patch
@workflow/nest	Patch
@workflow/rollup	Patch
@workflow/sveltekit	Patch
@workflow/vite	Patch
@workflow/nuxt	Patch
@workflow/ai	Patch

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[jcourson-bg]

Fixes #1374