chore: Add Iceberg TPC-H benchmarking scripts

[andygrove]

Summary

• Add scripts to benchmark TPC-H queries against Iceberg tables using Comet's native iceberg-rust integration
• create-iceberg-tpch.py: Convert Parquet TPC-H data to Iceberg tables
• tpcbench-iceberg.py: Run TPC-H queries against Iceberg catalog tables
• comet-tpch-iceberg.sh: Shell script to run the benchmark with Comet
• Update README.md with Iceberg benchmarking documentation

Test plan

• Run create-iceberg-tpch.py to create Iceberg tables from Parquet data
• Run comet-tpch-iceberg.sh and verify CometIcebergNativeScanExec appears in plans
• Compare benchmark results between Parquet and Iceberg formats

🤖 Generated with Claude Code

[andygrove]

@mbutrovich I can now run TPC-H w/ Iceberg native scan locally

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.95%. Comparing base (f09f8af) to head (07e1fa3).
⚠️ Report is 904 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3294      +/-   ##
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+ Coverage     56.12%   59.95%   +3.82%     
- Complexity      976     1473     +497     
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  Files           119      175      +56     
  Lines         11743    16167    +4424     
  Branches       2251     2682     +431     
============================================
+ Hits           6591     9693    +3102     
- Misses         4012     5126    +1114     
- Partials       1140     1348     +208     

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[andygrove]

Thanks @mbutrovich! I pushed a commit to address the feedback.