blog: add post on the Comet expression audit effort

[andygrove]

Summary

Add a blog post describing the systematic per-category audit of every supported Spark expression in DataFusion Comet against Spark 3.4.3, 3.5.8, 4.0.1, and 4.1.1.

The work produced 12 audit PRs (4469, 4470, 4473, 4474, 4475, 4476, 4478, 4479, 4480, 4483, 4486, 4493), one skill-update PR (4468), and 14 tracking issues for higher-risk findings (#4462–#4467, #4471–#4472, #4477, #4481–#4482, #4484–#4485, #4488–#4492).

The post covers:

• Why an expression audit matters (the four moving parts of a Comet expression that have to stay aligned)
• The audit-comet-expression Claude Code skill that drives the work
• The Step 5 consistency rules and the common antipatterns it catches
• What the Spark cross-version diff actually looks like in practice
• A sample of the 14 tracking issues filed
• The parallel-subagent dispatch pattern
• The skill-tightening PR that came out of the work

Marking this as a draft for review while the audit PRs land.

Test plan

• Render locally via make and verify formatting
• Check that all referenced PR / issue numbers resolve
• Confirm the post appears in the correct chronological place on the index