Documenting cache backends

[ksysoev]

This pull request improves the documentation and code comments for the layered and memcache backends, clarifying their behavior, caveats, and consistency guarantees. The changes provide a new backend comparison table in the README.md, add detailed explanations of each backend's semantics, and enhance inline comments for maintainability and onboarding.

Documentation improvements:

• Added a comprehensive backend comparison table and detailed sections to README.md, explaining TTL support, persistence, interoperability, and tradeoffs for each backend, as well as specific caveats for memcache and layered backends.

Layered backend clarifications:

• Updated comments in storage/layered/layered.go to clarify that operations are sequential (not transactional), partial failures are possible, and consistency is best effort/eventual. Also explained read, write, and delete semantics in detail. [1] [2] [3] [4] [5]

Memcache backend clarifications:

• Expanded comments in storage/memcache/memcache.go to document that values are always stored as protobuf envelopes (not raw bytes), explain TTL handling and interoperability caveats, and describe the rationale behind value encoding/decoding. [1] [2] [3] [4] [5] [6]

[Copilot]

Pull request overview

This pull request improves developer-facing documentation for the memcache and layered storage backends (and adds a README comparison table) to make their TTL semantics, value formats, and cross-layer failure/consistency behavior explicit for users and maintainers.

Changes:

• Added a backend comparison table and expanded backend-specific caveats/semantics sections in README.md.
• Clarified storage/memcache’s protobuf envelope storage format and TTL constraints in code comments.
• Clarified storage/layered’s sequential/non-transactional behavior, partial failure modes, and back-population error semantics in code comments.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
README.md	Adds a backend comparison table plus detailed caveats for memcache and semantics/failure modes for layered.
storage/memcache/memcache.go	Documents protobuf value-envelope encoding and TTL constraints/behavior used by GetWithTTL.
storage/layered/layered.go	Documents sequential cross-layer operations, partial-apply behavior, and back-population error semantics.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.22%. Comparing base (203c8b2) to head (3c7ca09).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

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  Coverage   88.22%   88.22%           
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  Files           9        9           
  Lines         501      501           
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  Hits          442      442           
  Misses         34       34           
  Partials       25       25           

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