fix(db): stamp the dialect into generated query files

The same generate command emitted ? one run and $1 the next, which looked
like non-determinism. It is not: postgres uses $1 placeholders where
sqlite and mysql use ?, and the driver comes from the active profile, so
building under a different profile rewrites this committed file.

The header now records the dialect it was generated for, making the flip
visible in the diff and explaining check:generated-types failures instead
of leaving them looking like random churn.

Worth deciding separately: a committed artifact whose contents depend on
the active profile will keep drifting. Either generate per dialect, or
stop committing it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/db/test/generate-dialect.test.ts

@@ -0,0 +1,23 @@
+import { expect, test } from "bun:test";
+import { generateQueriesFile, parseQueries } from "../src/generate.ts";
+
+const queries = parseQueries(`-- name: GetUser :one\nSELECT * FROM users WHERE email = :email;\n`);
+
+test("the generated header records the dialect it was built for", () => {
+  // The same command emits different SQL per dialect, so a build under another
+  // profile rewrites the committed file. The stamp makes that visible in the
+  // diff instead of looking like unexplained churn.
+  expect(generateQueriesFile(queries, [], "sqlite")).toContain("(dialect: sqlite)");
+  expect(generateQueriesFile(queries, [], "postgres")).toContain("(dialect: postgres)");
+});
+
+test("placeholder style follows the dialect", () => {
+  expect(generateQueriesFile(queries, [], "sqlite")).toContain("email = ?");
+  expect(generateQueriesFile(queries, [], "postgres")).toContain("email = $1");
+});
+
+test("generation is deterministic for a fixed dialect", () => {
+  const first = generateQueriesFile(queries, [], "postgres");
+  const second = generateQueriesFile(queries, [], "postgres");
+  expect(first).toBe(second);
+});