feat(indexing): support checkpoint progress writes

[yordis]

• Indexing processors need a shared progress boundary before lifecycle work can safely persist resume points.
• Persisting checkpoint progress separately from event materialization keeps the next lifecycle slices smaller and easier to validate.

[cursor]

PR Summary

Medium Risk
Checkpoint persistence affects indexing resume correctness, but this PR only adds the writer and tests without wiring it into live indexing paths yet.

Overview
Introduces IndexCheckpointWriter, a small coordinator on top of IIndexCheckpointStore so indexing can buffer resume progress and flush it on demand instead of writing on every event.

Track(ResolvedEvent) records commit/prepare positions from the event’s original position (requires OriginalPosition; otherwise throws). Repeated tracks keep the latest pending checkpoint under a lock. Commit no-ops when nothing was tracked; otherwise writes the pending checkpoint and clears it only if no newer track arrived during the async write. Read forwards to the store.

Adds IndexCheckpointWriterTests with fakes covering delegation, track/commit semantics, validation, and cancellation propagation.

^{Reviewed by Cursor Bugbot for commit f2fa2c9. 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: 17a54ebe-78e2-4cfb-9f20-7d7cb40c20b4

📥 Commits

Reviewing files that changed from the base of the PR and between 066ad50 and f2fa2c9.

📒 Files selected for processing (2)

• src/EventStore.Core.XUnit.Tests/Services/Storage/Indexing/IndexCheckpointWriterTests.cs
• src/EventStore.Core/Services/Storage/Indexing/IndexCheckpointWriter.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

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

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