docs: add React stale closure learning resource

[MuslumYilmaz]

Summary

• adds one advanced resource to the React section of the Learning Path
• covers stale closures, intentional snapshots, async races, and observable behavior across six code-review cases

Why

The current React list introduces components and hooks but has no advanced resource on callback lifetime and state freshness. This addition gives learners a focused next step without changing the existing progression.

Disclosure

I maintain FrontendAtlas. The linked clinic is public and requires no sign-up; separate exercises elsewhere on the site may be optional premium.

Verification

• confirmed the public URL loads
• documentation-only change; no local build was run

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