F3 SLICE-3 host fixes: disk-compare sync detection + self-minimized seam edits (INV-9) (#6)

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

test/pendingEdits.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { PendingEditRegistry } from "../src/pendingEdits";
+import { minimizeReplace, PendingEditRegistry } from "../src/pendingEdits";
 import type { Provenance } from "../src/model";
 
 const AGENT: Provenance = {
@@ -30,3 +30,22 @@ describe("PendingEditRegistry (INV-9)", () => {
     expect(reg.match("d.md", { start: 0, end: 0, text: "x" })).toBeNull();
   });
 });
+
+describe("minimizeReplace (host diff-minimization mirror)", () => {
+  it("trims a common suffix (the E2E-observed case)", () => {
+    expect(minimizeReplace("target sentence", "REWRITTEN-BY-CLAUDE sentence")).toEqual({ prefix: 0, suffix: 9 });
+  });
+  it("trims a common prefix", () => {
+    expect(minimizeReplace("Hello world", "Hello there")).toEqual({ prefix: 6, suffix: 0 });
+  });
+  it("trims both, never overlapping", () => {
+    expect(minimizeReplace("aba", "aa")).toEqual({ prefix: 1, suffix: 1 });
+    expect(minimizeReplace("aaaa", "aa")).toEqual({ prefix: 2, suffix: 0 });
+  });
+  it("identical texts minimize to nothing", () => {
+    expect(minimizeReplace("same", "same")).toEqual({ prefix: 4, suffix: 0 });
+  });
+  it("disjoint texts trim nothing", () => {
+    expect(minimizeReplace("abc", "xyz")).toEqual({ prefix: 0, suffix: 0 });
+  });
+});