Rewritten LRU with safer flows, explicit APIs, and improved cleanup handling

[Nondukishor]

Rewritten LRU with safer flows, explicit APIs, and improved cleanup handling

Summary:

This PR rewrites the internal LRU cache implementation to improve safety, maintainability, and predictable behavior. The previous implementation relied on implicit assumptions about node validity, list integrity, and scheduler availability. This updated version strengthens those areas and introduces a more explicit and robust API.

Key Improvements

• Stricter Flow types for Entry and the public LRU interface.
• Stronger doubly-linked-list invariants with defensive guards against corrupted or deleted entries.
• Safe fallback scheduling when React Scheduler is unavailable.
• More reliable cleanup behavior, ensuring that onDelete errors do not interrupt eviction.
• Explicit and expanded APIs, including:
  • add
  • access
  • update
  • remove
  • setLimit
  • dump
  • getSize
  • getLimit
• Immediate cleanup scheduling within add() to prevent uncontrolled memory growth.
• Frozen public API object to avoid accidental mutation.
• Improved GC friendliness by fully detaching deleted nodes.
• Enhanced debugging utilities through dump() and size helpers.

Why this change?

• While functional, the earlier implementation had several limitations:
• Accessing or updating deleted entries could happen silently.
• Corrupted circular list states were not detected.
• Errors thrown inside onDelete could interrupt eviction.
• No direct method existed to remove an entry explicitly.
• Lack of scheduler fallback.
• Debugging and inspecting internal cache state was difficult.
  This rewrite improves the resilience of the LRU cache and ensures predictable cleanup behavior even in error scenarios.

##How did you test this change?

• Manually tested list behavior including add, access, update, and remove.
• Verified correct eviction ordering by exceeding cache limits.
• Simulated both presence and absence of the React Scheduler to confirm fallback logic.
• Ensured errors thrown inside onDelete do not break cleanup.
• Verified deleted entries cannot be accessed or updated.

Ran the full test suite:
yarn test
yarn test --prod

Verified Flow type checks:
yarn flow

Verified formatting and linting:
yarn prettier
yarn lint / yarn linc

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