fix(catalog): synthesize Postgres system columns (xmin, ctid, ...) (#3742)

[luongs3]

Problem

Fixes #3742.

sqlc errors with column "..." does not exist when a PostgreSQL query references any of the six system columns (tableoid, xmin, cmin, xmax, cmax, ctid) unless database.managed: true is configured.

The earlier fix (#2871 / #1745) only added these columns inside the PostgreSQL analyzer, which is initialized only when a live database is attached. Code-only pipelines that build the catalog purely from schema files still hit the same failure 17 months later. The issue thread on #3742 confirms this — swallowstalker already diagnosed it, and werjo-gridfuse noted that requiring a live DB doesn't work for code-gen pipelines.

Repro on main (no live DB):

CREATE TABLE authors (id BIGSERIAL PRIMARY KEY, name text NOT NULL, bio text);
-- query.sql
SELECT xmin, ctid FROM authors;

sql/query.sql:2:8: column "xmin" does not exist

Fix

Synthesize the six PostgreSQL system columns inside compiler.QueryCatalog.GetTable whenever the engine is postgresql. The catalog itself is left untouched (so MySQL/SQLite tables are not polluted), and the synthesized columns are marked with a new Column.IsSystem flag so SELECT * / RETURNING * expansion skips them — matching PostgreSQL's own behavior. Explicit references like SELECT xmin, ctid FROM foo and aliased forms like a.xmin resolve to the correct types (oid/xid/cid -> pgtype.Uint32, tid -> pgtype.TID).

Changed files:

• internal/compiler/query.go — add Column.IsSystem.
• internal/compiler/query_catalog.go — track engine on QueryCatalog; append pgSystemColumns in GetTable when engine is postgresql.
• internal/compiler/expand.go — skip IsSystem columns in * expansion.
• internal/compiler/output_columns.go — skip IsSystem columns in the output-columns * branch (mirrors expand.go).
• internal/endtoend/testdata/system_columns_3742/postgresql/pgx/v5/ — new fixture exercising all six system columns, a table alias, and SELECT * (which must omit them). Golden files committed.

Tests

go test ./internal/compiler/... ./internal/sql/... ./internal/engine/... ./internal/codegen/...   # all pass
go test -run TestExamples ./internal/endtoend/                                                    # pass
go vet ./...                                                                                       # clean
gofmt -l <touched files>                                                                           # clean

Local end-to-end verification against the exact schema + query from the issue:

• Before fix: EXIT=1, sql/query.sql:2:8: column "xmin" does not exist
• After fix: EXIT=0, generates pgtype.Uint32 for xmin, pgtype.TID for ctid. SELECT * still expands to id, name, bio only.

What does NOT change

• MySQL and SQLite catalogs are untouched.
• The managed-database analyzer path (feat(postgresql): Support system columns on tables #2871) is unchanged and still works for users on database.managed: true.
• SELECT * continues to omit system columns (matches PostgreSQL behavior).
• The existing select_system end-to-end fixture (managed-db) is left alone; the new fixture covers the non-managed code path that was broken.