Add checksum to S3 Anchor and change clone template as file-constant

[JasMehta08]

This Pull request:

(Is a part of the GSoC 2026 project S3 Backend for RNTuple.)

Adds checksum and a configurable clone path to the S3 Anchor.

Changes or fixes:

• Add an XXH3 64-bit checksum to the S3 anchor, computed over a canonical compact JSON representation of all data fields. Verified on read.
• Replace the hardcoded clone path in CloneAsHidden with a file-local constant (kDefaultCloneTemplate).

Checklist:

• tested changes locally

[JasMehta08]

@silverweed and @jblomer

I have removed the clone template from the anchor, and made it a file-constant, please have a new review of the commits

[silverweed]

Aren't we duplicating quite a bunch of work here? I get we need to serialize the anchor to compute the checksum and then add it to the JSON representation, but can't we add the checksum via string manipulation? (replacing canonical's last } with ,"checksum":...).

[JasMehta08]

Yes, we can remove the use of CanonicalPayload from ToJSoN(), if we can just use the jsonAnchor to create the checksum and add the field at the end. I will update it to do so.

[github-actions]

Test Results

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Results for commit 5945e72.

[JasMehta08]

@silverweed ,

I have updated the implementation, and removed CanonicalPayload() from both ToJSoN() as well as CreateFromJSoN(). I removed the duplicate work that was being done by it. ToJSoN() using the anchor itself to create and checksum, and attaches it at the end. In CreateFromJSoN() it strips the checksum of the anchor, then using the remaining anchor to create a new hash and check against the checksum.