Refactor SentenceAwareChunker chunking logic, bump Swift tools, and pin Xcode in CI

[joelnishanth]

Motivation

• Fix incorrect/fragile sentence-based chunking and improve handling of word counts and overlaps when building chunks.
• Ensure the package uses the newer Swift toolchain features by updating the package tools version to 6.1.
• Stabilize CI by pinning the Xcode version used in GitHub Actions to avoid variations from latest-stable.

Description

• Rewrote the chunking loop in SentenceAwareChunker to use sentence indices and accumulate chunkWordCount with explicit start/end indexes, producing consistent sourceWordRange and metadata for each TextChunk.
• Ensure chunks respect targetWords, enforce at-least-one-sentence per chunk, and implement sentence overlap rewind using overlapSentences to compute the next sentenceIndex correctly.
• Updated // swift-tools-version in Package.swift from 5.9 to 6.1 to adopt the newer Swift toolchain.
• Pinned the GitHub Actions Xcode selection to 16.3 in .github/workflows/ci.yml instead of latest-stable.

Testing

• Ran swift test against the package and the test suite completed successfully.
• CI workflow updated to run on macos-14 with Xcode 16.3 and will execute swift test as part of checks.

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Codex Task