Fix SwiftPM portability, sentence splitting, CI, and README accuracy

[joelnishanth]

Motivation

• Make the package buildable and testable across environments without requiring the MLX runtime dependency and fix Linux-compatible sentence splitting that was breaking builds.
• Ensure CI runs both swift build and swift test on a supported macOS runner.
• Clarify README claims about coverage and metadata extraction to avoid overpromising.

Description

• Remove the hard dependency on mlx-swift-lm from Package.swift so the core library is backend-agnostic and keeps product MLXSwiftChain and package name mlx-swift-chain intact.
• Make MLXBackend conditionally available using #if canImport(MLXLMCommon) so MLX-specific integration does not block builds where MLXLMCommon is absent.
• Replace String sentence enumeration using unavailable .bySentences with a regex-based splitter in SentenceAwareChunker to support swift-corelibs-foundation on Linux.
• Remove the invalid @_exported import struct Foundation.Duration re-export that prevented compilation.
• Update CI workflow .github/workflows/ci.yml to run on macos-latest and execute both swift build and swift test.
• Update README.md to clarify that the “100%” figures refer to model-visible/input coverage across chunks (not guaranteed final-answer retention), and note that timestamp/speaker-label discovery is lightweight regex-based extraction rather than a full diarization pipeline; update requirements to Swift 6.1+ and note MLXBackend availability.

Testing

• Ran swift package describe and it completed successfully.
• Ran swift build locally and the build completed successfully.
• Ran swift test locally and all automated tests passed (28 tests).

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Codex Task