fix(python): preserve signed code-object line numbers

Fixes #6490

src/python/PythonPyc.fixture.ts

@@ -8,6 +8,11 @@ const u32 = (value: number): Buffer => {
   bytes.writeUInt32LE(value);
   return bytes;
 };
+const i32 = (value: number): Buffer => {
+  const bytes = Buffer.alloc(4);
+  bytes.writeInt32LE(value);
+  return bytes;
+};
 const concat = (...parts: Buffer[]): Buffer => Buffer.concat(parts);
 
 /** 16-byte CPython 3.7+ pyc header. Hash-based pycs store an 8-byte source hash. */
@@ -136,7 +141,7 @@ export const marshalCode = (
         marshalShortAscii(fields.qualname ?? fields.name ?? "<module>"),
     );
   parts.push(
-    u32(fields.firstlineno ?? 1),
+    i32(fields.firstlineno ?? 1),
     marshalBytes(fields.linetable ?? Buffer.alloc(0)),
   );
   if (layout === "py311")

src/python/PythonPycParser.test.ts

@@ -72,6 +72,46 @@ it("reads 3.7, 3.10 and 3.12 layouts including nested code", () => {
   );
 });
 
+it("preserves signed co_firstlineno in code objects and code constants", () => {
+  const nested = marshalCode("py311", {
+    name: "inner",
+    firstlineno: -7,
+  });
+  const bytes = Buffer.concat([
+    pythonPycHeader(3531),
+    marshalCode("py311", {
+      firstlineno: -1,
+      consts: [nested],
+    }),
+  ]);
+  const report = parse(bytes);
+  expect(root(report).firstlineno).toBe(-1);
+  expect(root(report).consts).toContainEqual({
+    kind: "code",
+    name: "inner",
+    firstlineno: -7,
+  });
+  expect(pythonPycSchema.safeParse(report).success).toBe(true);
+  expect(
+    pythonPycSchema.safeParse({
+      ...report,
+      code_objects: [
+        { ...root(report), firstlineno: -0x8000_0001 },
+        ...report.code_objects.slice(1),
+      ],
+    }).success,
+  ).toBe(false);
+  expect(
+    pythonPycSchema.safeParse({
+      ...report,
+      code_objects: [
+        { ...root(report), firstlineno: 0x8000_0000 },
+        ...report.code_objects.slice(1),
+      ],
+    }).success,
+  ).toBe(false);
+});
+
 it("decodes a hash-based header and a FLAG_REF top-level code object", () => {
   const bytes = pythonPycFixture("py311", {
     flaggedCode: true,

src/python/PythonPycParser.ts

@@ -457,7 +457,7 @@ class MarshalReader {
       this.layout === "py311"
         ? asString(this.readObject(depth + 1), "co_qualname")
         : null;
-    const firstlineno = this.u32();
+    const firstlineno = this.i32();
     const linetable = asBytes(this.readObject(depth + 1), "co_linetable");
     const exceptiontable =
       this.layout === "py311"