[xen0l]

Fixes #4658

Problem

The preprocess dialect table (internal/sql/preprocess/dialect.go) had no entry for DuckDB, and an engine with no entry gets its source back unchanged. sqlc.arg(), sqlc.narg() and @name then reached the analyzer as function calls and column names (unknown column "want_id"), or failed in the parser (@).

Change

• Add a config.EngineDuckDB entry: $1 placeholders, @name as sqlc syntax, $$ strings, $1 as a bind parameter. A bare ? is left to the engine, as before.
• Add a NoSlice dialect flag and set it for DuckDB. sqlc.slice is rejected with sqlc.slice is not supported by this engine; bind a list parameter instead.
• Add a DuckDB row to the dialect table in internal/sql/preprocess/CLAUDE.md.

Why sqlc.slice is rejected: with $1 placeholders the generated Go still calls strings.Replace on a ? marker that the SQL does not contain. The call changes nothing, so the query binds only the first element of the slice and returns wrong rows without an error. I ran id IN (sqlc.slice(ids)) with []int32{1, 3} on DuckDB and got one row. A DuckDB user binds a list with x = ANY($1::INTEGER[]) or list_contains($1::INTEGER[], x); I ran both with []int32{1, 3} and each returned the right two rows.
The PostgreSQL engine generates the same IN ($1) and the same no-op Replace today. I did not change that here.

Tests

New end-to-end cases:

• duckdb_named_params/duckdb (sqlc generate, output committed): sqlc.arg, sqlc.narg, @name, and two sqlc.arg in one query. It fails without the dialect entry (Parser Error: syntax error at or near "@") and passes with it.
• analyze_slice_rejected/duckdb (sqlc analyze, stderr.txt committed): sqlc.slice must fail. It fails without the NoSlice flag and passes with it.

go test ./internal/endtoend -run TestReplay/base: 971 pass, 58 skip, 0 fail before; 973 pass, 58 skip, 0 fail after. The only differences are the two new cases.
go test ./internal/engine/... ./internal/core/... ./internal/compiler/... ./internal/codegen/... ./internal/cmd/... ./internal/metadata/... ./internal/sql/...: all ok.

I ran the generated Go of the first case on DuckDB v1.5.6 (duckdb-go/v2 v2.10506.0): ByID(2), ByAt(3), ByNullableID(4) and Range(1,4) each returned the expected rows.

Not run: the managed-db context, --tags=examples, and the internal/goldeneye checks.

Notes

• A query that mixes sqlc.arg(a) with an explicit $2 fails with could not determine data type of parameter $1. The PostgreSQL and SQLite engines give the same error for the same mix, so I left it.
• I set the lexical flags from DuckDB's documented syntax. I did not set NestedBlockComment, because I could not confirm whether the DuckDB parser nests block comments. Please check it.