feat(indexing): support in-memory index reads

[yordis]

• Indexing-owned streams need a local read boundary before lifecycle wiring can safely expose concrete components.
• Keeping paging and metadata behavior covered in isolation reduces risk when later components begin publishing index records.

[cursor]

PR Summary

Low Risk
New isolated indexing storage components with no production wiring in this diff; behavior is covered by unit tests and does not alter existing read paths until integrated.

Overview
Adds an in-memory read path for indexing-owned streams: a thread-safe InMemoryIndexEventBuffer that accepts sequentially numbered index events for a single IndexStreamId, and InMemoryIndexVirtualStreamReader, which implements forward/backward stream reads and last-event / last-indexed metadata from buffer snapshots.

Read behavior matches existing virtual-stream semantics (empty stream → NoStream, paging, negative from handling, end-of-stream flags, TfLastCommitPosition). The reader plugs into the existing VirtualStreamReader composite via IndexVirtualStreamReader.

XUnit coverage exercises buffer validation (null args, stream id mismatch, gap in event numbers) and reader edge cases, including routing through VirtualStreamReader.

^{Reviewed by Cursor Bugbot for commit 3498ebc. 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: 14ce007d-5691-44a5-b897-f8321aa8aac9

📥 Commits

Reviewing files that changed from the base of the PR and between cda6d10 and 3498ebc.

📒 Files selected for processing (3)

• src/EventStore.Core.XUnit.Tests/Services/Storage/Indexing/InMemoryIndexVirtualStreamReaderTests.cs
• src/EventStore.Core/Services/Storage/Indexing/InMemoryIndexEventBuffer.cs
• src/EventStore.Core/Services/Storage/Indexing/InMemoryIndexVirtualStreamReader.cs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/feat-index-in-memory-reader

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