Update README with current implementations and prerequisites

[SoraSuegami]

Summary

• Rewrote the README to describe the current lattice-crypto scope of mxx and the major implementation areas in the repo.
• Added a paper-to-directory mapping for BGG+, WEE25 matrix commitment, BGG+ lookup-table evaluation, GSW-FHE evaluation/decryption, AKY24-based benchmark estimation, and Diamond iO benchmark estimation.
• Updated prerequisites and Cargo feature notes, including the OpenFHE fork requirement and GPU/CUDA build expectations.

Testing

• Not run (not requested)
• Performed documentation-only validation during editing: diff whitespace checks, path checks, and metadata-level review of the referenced features and directories.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Restrict the unit-test command to the library target

I checked cargo test --help; it describes cargo test as executing all unit and integration tests, and lists --lib as testing only the package library. In this repo, integration tests are explicitly gated unless requested, so documenting cargo test <test_name> under “targeted unit test” can still compile and run matching tests from tests/; use cargo test --lib <test_name> or a specific --test command only when integration tests are approved.

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