Cherry-pick upstream #1234: fix add-files partition key lookup for repeated transforms

[EDsCODE]

Problem

ducklake_add_data_files() writes every ducklake_file_partition_value row at the last transform's partition_key_index when a partition spec applies multiple transforms to one source column — e.g. the PostHog duckling backfill's events spec year(timestamp), month(timestamp), day(timestamp). The lookup map was keyed on the bare source field index, so the three spec entries collapsed to one.

Symptoms downstream:

• Tier-3 compaction fails with DuckLakeCompactor: Files have different hive partition path.
• Partition pruning silently misses files.

PostHog worked around this in the Dagster backfill (PostHog/posthog#67168) with a post-register catalog fix-up, but that fix-up needs a direct catalog Postgres connection that CR-based (CNPG-shard) ducklings can't provide from Dagster, so backfills on prod-us/prod-eu currently run with the fix-up disabled (PostHog/charts#13004) and accumulate mis-stamped rows.

Changes

Cherry-picks upstream duckdb/ducklake#1234 (both commits, -x provenance preserved; zero conflicts):

• a5a6be4d — key the partition lookup on (field index, transform) instead of bare field index; regression test with year/month/day on one timestamp column in add_file_partitioned.test.
• fda045ba — refactor: each parsed HivePartition records its partition_key_index at map time and validation compares the full transform (handles repeated bucket transforms with different widths too); new add_file_repeated_bucket.test.

Plus a docs commit recording the cherry-pick in the "PostHog changes" list.

Testing

• just build — clean (732/732 targets; local build needs croaring on CMAKE_PREFIX_PATH, e.g. brew install croaring).
• ./build/release/test/unittest "test/sql/add_files/*" — 34/34 test cases, 1134 assertions, including both new regression tests.
• just test (full suite) — all pass: 404 test cases, 14724 assertions, 18 environment-gated skips (postgres_scanner/spatial/sqlite_scanner extensions and CI-only env vars).

Release / follow-up

After merge: tag v1.0-posthog.5 (release binaries publish on tag push), bump DUCKLAKE_EXTENSION_TAG in duckgres Dockerfile + Dockerfile.worker, roll images. Then revert PostHog/charts#13004 and re-run registration on partitions registered while the fix-up was off (the fix-up's DELETE-then-INSERT is idempotent and convergent, so re-registration heals the rot).