api: polish FetchKitLibrary caller surface

[gaelic-ghost]

Summary

• make batch removal read as removeDocuments(withIDs:)
• return affected document IDs from removeAllDocuments() and add empty-result helpers
• give index-sync failures a localized message and close the Milestone 4 roadmap loop

Verification

• swift package dump-package
• swift test --filter FetchKitLibrary
• scripts/repo-maintenance/validate-all.sh

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: d4447b85-bda3-4c91-9e33-c5f8744f5302

📥 Commits

Reviewing files that changed from the base of the PR and between a31dd02 and 6ebff81.

📒 Files selected for processing (5)

• README.md
• ROADMAP.md
• Sources/FetchKit/FetchKitLibrary.swift
• Tests/FetchKitTests/FetchKitLibraryTests.swift
• docs/maintainers/fetchkit-product-plan.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch api/fetchkit-library-polish

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