feat(storage): add variant shredding for parquet

[dantengsky]

I hereby agree to the terms of the CLA available at: https://docs.databend.com/dev/policies/cla/

Summary

• Enable Parquet VARIANT encoding + inline shredding for Variant columns in Fuse
• Add read-path unshred, stats gating, and EXPLAIN inline indication with JSONB compatibility
• Add unit + sqllogic coverage for typed_value decoding and inline shredding

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[github-actions]

🤖 CI Job Analysis

Workflow: 21800558292

📊 Summary

• Total Jobs: 85
• Failed Jobs: 2
• Retryable: 0
• Code Issues: 2

❌ NO RETRY NEEDED

All failures appear to be code/test issues requiring manual fixes.

🔍 Job Details

• ❌ linux / test_private_tasks: Not retryable (Code/Test)
• ❌ linux / sqllogic / ee (parquet): Not retryable (Code/Test)

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🤖 About

Automated analysis using job annotations to distinguish infrastructure issues (auto-retried) from code/test issues (manual fixes needed).