[RNTuple] add LZ4 compression for RNTuple writing and default to it

[Moelf]

this goes on top of #438 (I don't have Stacked PR so no UI sugar :()

[Copilot]

Pull request overview

This PR adds ROOT-style write-time compression for UnROOT’s RNTuple writer, defaulting output to LZ4 (ROOT fCompress 404), and expands CI validation to cross-check written files with C++ ROOT via a data-driven sidecar spec.

Changes:

• Implement ROOT compression block framing for RNTuple writing (LZ4/ZSTD/ZLIB) and apply it to header/footer/page-link envelopes and per-page payloads.
• Default write_rntuple(...; compression=...) to LZ4 (404) and propagate fCompress into the ROOT file header.
• Replace the previous ad-hoc ROOT validation script with a JSON-driven validator and run it in CI for multiple compression settings.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/RNTuple/Writing/compression.jl	Adds ROOT block framing and per-algorithm compression helpers for the writer.
src/RNTuple/Writing/TFileWriter.jl	Plumbs compression through writing, compresses envelopes/pages, and records on-disk sizes in locators/anchor.
src/UnROOT.jl	Imports zlib compression APIs and includes the new compression implementation.
test/RNTupleWriting/highlevel.jl	Adds round-trip tests across compression modes and checks default LZ4 behavior/size impact.
test/RNTupleWriting/output_sample.jl	Generates deterministic sample data plus a JSON sidecar describing expected values and compression.
test/RNTupleWriting/validate_rntuple.py	New ROOT-based validator that checks readability and exact value correctness from the JSON sidecar.
test/RNTupleWriting/test1.py	Removes the old minimal validation script.
.github/workflows/cvmfs.yml	Updates CI to generate/validate files for multiple compression settings using C++ ROOT.

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[tamasgal]

Let's go ;)