README + CI consistency cleanup

[joelnishanth]

Motivation

• Align documentation with the package toolchain (// swift-tools-version: 6.1) and avoid implying perfect semantic retention in the reduced output.
• Make CI validate the local Swift environment and package lifecycle (describe, build, test) on the same macOS runner used previously.

Description

• Update README Requirements from Swift 5.9+ to Swift 6.1+ to match Package.swift.
• Rename the benchmark table column from mlx-swift-chain Coverage to mlx-swift-chain Model-Visible Coverage.
• Add the sentence under the benchmark table: Coverage here means input/model-visible coverage across chunks, not guaranteed perfect retention in the final reduced answer.
• Expand .github/workflows/ci.yml to run swift --version, swift package describe, and swift build before swift test, and keep runs-on: macos-15 unchanged.
• Leave Package.swift and package architecture unchanged.

Testing

• Ran swift --version, which succeeded and reported the installed Swift version.
• Ran swift package describe, which succeeded and produced the package description.
• Ran swift build, which failed in this environment due to a dependency fetch error (HTTP 403) when cloning https://github.com/ml-explore/mlx-swift-lm.
• Ran swift test, which also failed for the same dependency fetch error in this environment.

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Codex Task