Add checksum and clone template to S3 Anchor

[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.
• Add fCloneTemplate field to RNTupleAnchorS3 for resolving attribute-set (hidden) ntuple base URLs. Serialized into the anchor JSON and extracted on read (optional, for forward compatibility with older anchors).

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

[github-actions]

Test Results

    23 files      23 suites   3d 17h 30m 12s ⏱️
 3 874 tests  3 874 ✅ 0 💤 0 ❌
78 880 runs  78 880 ✅ 0 💤 0 ❌

Results for commit e38221d.

♻️ This comment has been updated with latest results.

[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.

[silverweed]

Thanks! When @jblomer approves we can merge

[JasMehta08]

@jblomer and @silverweed

I have added the cloneTemplate into the in the anchor for now, for backwards compatibility. I have not made changes to CreateFromJSON() as it would require adding an additional field to the anchor struct, and will add it with the reader changes. Or would it be better to just add it now?

[JasMehta08]

@silverweed and @jblomer

there was a small issue with the clang formatting that I have fixed, the other failures dont seem to be related to changes due to this PR.

[jblomer]

LGTM, one tiny comment.