F2 SLICE-4: lock reload/re-anchor/orphan decision logic with unit tests (#4)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/reanchor.test.ts

@@ -0,0 +1,28 @@
+import { describe, it, expect } from "vitest";
+import { buildFingerprint, resolve } from "../src/anchorer";
+
+const ORIGINAL = "intro\nThe quick brown fox\noutro\n";
+
+describe("re-anchor scenarios (SLICE-4 decision logic)", () => {
+  it("reload, document unchanged: resolves to the same range", () => {
+    const start = ORIGINAL.indexOf("quick brown");
+    const fp = buildFingerprint(ORIGINAL, { start, end: start + "quick brown".length });
+    expect(resolve(ORIGINAL, fp)).toEqual({ start, end: start + "quick brown".length });
+  });
+
+  it("external edit that moves the anchored text down: re-resolves to the new range", () => {
+    const start = ORIGINAL.indexOf("quick brown");
+    const fp = buildFingerprint(ORIGINAL, { start, end: start + "quick brown".length });
+    const edited = "a new first line\nanother line\n" + ORIGINAL;
+    const newStart = edited.indexOf("quick brown");
+    expect(newStart).not.toBe(start);
+    expect(resolve(edited, fp)).toEqual({ start: newStart, end: newStart + "quick brown".length });
+  });
+
+  it("external edit that deletes the anchored text: orphaned, never moved (INV-1)", () => {
+    const start = ORIGINAL.indexOf("quick brown");
+    const fp = buildFingerprint(ORIGINAL, { start, end: start + "quick brown".length });
+    const edited = "intro\nThe lazy dog\noutro\n";
+    expect(resolve(edited, fp)).toBe("orphaned");
+  });
+});