Validate CSV output schemas

Also moved code dir to src.
There are reasons to do standard things in standard ways. While it is
possible to get the `code/` directory to work, and recognize it as a
package path, this requires wrangling the pyproject.toml file.
Additionally, any import from the `code.something` path automatically
shadows the python stdlib `code` module. While it may not be necessary,
it still is good to not shadow standard library modules.

src/tests/test_validate_unique_installs.py

@@ -0,0 +1,24 @@
+import dataframely as dy
+import polars as pl
+
+from tests.test_validate_date_col import DateSchema
+
+
+class UniquesSchema(DateSchema):
+    unique: dy.Integer = dy.Integer(nullable=False)
+
+    @dy.rule()
+    def cannot_be_zero() -> pl.Expr:
+        return pl.col("unique") > 0
+
+
+def test_uniques_schema():
+    _ = UniquesSchema.validate(
+        pl.scan_csv(
+            "output/unique_installs.csv",
+            schema={
+                "date": pl.Date,
+                "unique": pl.UInt16,
+            },
+        ).collect(engine="streaming")
+    )