Upgrade bundled DuckDB to 1.5.5

[skuirrels]

Summary

• upgrade bundled native DuckDB from v1.5.4 to v1.5.5 for package version 1.5.5.2
• align statement-type ABI values and add v1.5.5 compatibility coverage for MERGE INTO and SHOW TABLES
• invalidate native caches by exact artifact URL so incremental builds cannot retain v1.5.4
• keep the fork package release workflow stable-only

Benchmark

All 23 comparisons completed against the same managed source with native v1.5.4 versus v1.5.5. The candidate is broadly performance-neutral: TPC-H, materialization, appender ingestion, command construction, and unprepared paths show no clear difference at the 99.9% confidence-interval level. Array-backed list append paths improved by 6.64% and 8.97%; several small prepared operations were 2.61% to 4.84% slower in this single sequential local run.

Verification

• Release build: passed
• .NET 8: 7,021 tests passed
• .NET 10: 7,021 tests passed
• package smoke tests passed on .NET 8 and .NET 10 with runtime v1.5.5
• all five packaged native RIDs verified as v1.5.5
• exact package checksums and clean source provenance recorded locally