feat(indexing): support checkpoint storage

[yordis]

• Indexing components need a tested checkpoint boundary before lifecycle wiring can safely resume work.
• Keeping checkpoint state isolated reduces risk when future processors begin committing progress.

[cursor]

PR Summary

Low Risk
New isolated abstraction and tests only; no changes to production wiring or durable checkpoint paths in this diff.

Overview
Introduces IIndexCheckpointStore with async Read / Write for IndexCheckpoint (commit/prepare positions), plus a thread-safe InMemoryIndexCheckpointStore that honors cancellation and overwrites on write.

Adds xUnit coverage for empty read, round-trip, overwrite, and cancelled read/write. This is foundational storage for indexing progress; the diff does not wire it into node lifecycle or durable persistence yet.

^{Reviewed by Cursor Bugbot for commit 6aa8fd2. Bugbot is set up for automated code reviews on this repo. Configure here.}

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 9fd2cdc3-c339-4c80-8a58-e68e8a90f524

📥 Commits

Reviewing files that changed from the base of the PR and between 9cd4d2b and 6aa8fd2.

📒 Files selected for processing (3)

• src/EventStore.Core.XUnit.Tests/Services/Storage/Indexing/InMemoryIndexCheckpointStoreTests.cs
• src/EventStore.Core/Services/Storage/Indexing/IIndexCheckpointStore.cs
• src/EventStore.Core/Services/Storage/Indexing/InMemoryIndexCheckpointStore.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/feat-index-checkpoint-store

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