test(guards): let the file-size floor follow the ratchet down

The splits on this line took the files over 800 lines from 115 to 96,
under the liveness floor of 100. The floor guards the matcher, so it
moves to 80, below the real count.

src/__tests__/source-size-ratchet.test.ts

@@ -92,7 +92,10 @@ describe("source size ratchet", () => {
   it("reads the whole tree", () => {
     expect(m.files).toBeGreaterThan(3000);
     expect(m.functionsSeen).toBeGreaterThan(20_000);
-    requireFloor("files over the cap", Object.keys(m.fileLines), 100);
+    // The floor follows the ratchet down: 115 files sat over the cap before
+    // the large-component splits, 96 after. It guards the matcher, so it sits
+    // under the real count, not at it.
+    requireFloor("files over the cap", Object.keys(m.fileLines), 80);
     requireFloor("functions over the cap", Object.keys(m.functionLines), 300);
   });