Add a DuckDB engine (PostgreSQL query set, local reference)

[litkhai]

Closes #10

What

DuckDB as a sixth engine, wired like the others. Its query set is the PostgreSQL query set of this
repository, so it can serve as a local reference for engines that parse PostgreSQL SQL.

• tools/sync-upstream.sh duckdb — PostgreSQL text + a DuckDB-only fix only for the two queries that fail on DuckDB.
  The script aborts if a fix stops changing its query or if an unfixed query drifts from the PostgreSQL text.
• tools/derive-ddl.sh duckdb — same types as PostgreSQL; DuckDB accepted every one.
• engines/duckdb/load/load.sh — COPY per .dat file; row count checked against the file line count; CHECKPOINT.
• bin/ — duckdb in ENGINES, engine_exec (-bail, -nullvalue '', optional DUCKDB_THREADS / DUCKDB_MEMORY_LIMIT), check_client, ddl.sh / load.sh / run.sh help.
  bin/run.sh no longer uses mapfile, so it runs on macOS bash 3.2 (the duckdb client runs on the host).
• tools/verify.sh --engine duckdb — host CLI + temporary database file, no docker; added to --all.
• CI — duckdb in the query-count loop and in the engine matrix; CLI pinned to v1.5.6, duckdb_cli-linux-amd64.zip checked against the release asset digest. No new job.
• Docs — docs/engines/duckdb.md, engine index, nav, README, NOTICE, quickstart, commands, licensing, verification page.

Compatibility of the PostgreSQL files on DuckDB (measured 2026-10-09, DuckDB v1.5.6, macOS arm64, fixture from tools/make-fixture.py)

101 of 103 engines/postgres/queries/*.sql run unchanged. The two that do not fail at parse time:

Query	First error	DuckDB-only fix
q77	Parser Error: syntax error at or near "returns"	coalesce(returns, 0) returns -> coalesce(returns, 0) as returns
q90	Parser Error: syntax error at or near "at"	) at, -> ) as "at",

Full per-query table: docs/engines/duckdb.md. "Runs" = parse, bind and execute with exit status 0; 47 of the 103 return zero rows on this fixture, and no answer was compared.

Verification

tools/verify.sh --all on 2026-10-09: postgres 103/103, clickhouse 102/103 (+1 known), starrocks 103/103, duckdb 103/103 (also in results/verification.md).
Fault injection: a broken from date_dim_missing in engines/duckdb/queries/query42.sql made verify.sh --engine duckdb report 102/103 (1 unexpected) and q42; a stale q77 pattern in the generator aborts with "pattern is stale"; a missing duckdb binary fails with a clear message.

Not run

• The CI workflow (this PR's first run is the first run; the pinned Linux asset's sha256 and zip layout were checked by hand, the step itself was not executed).
• Any scale factor above the fixture (trailing-delimiter behaviour, load time, memory, primary-key cost on real dsdgen output).
• tools/verify-docs.sh (browser check; tools/verify-docs.js hardcodes its engine list and was not touched, so DuckDB is not covered by it).

Delegated-work cost (cost.py): not measured.

🤖 Generated with Claude Code