fix(FN-XXX): harden windows path handling

packages/core/src/__tests__/pi-extensions-format.test.ts

@@ -0,0 +1,38 @@
+import { describe, expect, it } from "vitest";
+import { formatPiExtensionSource } from "../pi-extensions.js";
+
+describe("formatPiExtensionSource", () => {
+  it("formats project-relative extension paths", () => {
+    expect(
+      formatPiExtensionSource(
+        "fusion-project",
+        "/repo/.fusion/extensions/tooling/index.ts",
+        "/repo",
+        "/Users/alice",
+      ),
+    ).toBe("fusion-project: .fusion/extensions/tooling/index.ts");
+  });
+
+  it("formats home-relative Windows extension paths", () => {
+    expect(
+      formatPiExtensionSource(
+        "fusion-global",
+        "C:\\Users\\alice\\.fusion\\agent\\extensions\\tooling\\index.ts",
+        "C:\\repo",
+        "C:\\Users\\alice",
+      ),
+    ).toBe("fusion-global: ~/.fusion/agent/extensions/tooling/index.ts");
+  });
+
+  it("does not treat prefix-matching sibling paths as inside the project or home", () => {
+    const projectLookalike = "C:\\repo-two\\.fusion\\extensions\\tooling\\index.ts";
+    const homeLookalike = "C:\\Users\\alice-dev\\.fusion\\agent\\extensions\\tooling\\index.ts";
+
+    expect(
+      formatPiExtensionSource("fusion-project", projectLookalike, "C:\\repo", "C:\\Users\\alice"),
+    ).toBe(`fusion-project: ${projectLookalike}`);
+    expect(
+      formatPiExtensionSource("fusion-global", homeLookalike, "C:\\repo", "C:\\Users\\alice"),
+    ).toBe(`fusion-global: ${homeLookalike}`);
+  });
+});

packages/core/src/__tests__/setup-test-isolation.test.ts

@@ -8,10 +8,12 @@ const TEMP_HOME_PREFIX = "fn-test-home-";
 describe("test isolation setup", () => {
   it("process.env.HOME is overridden to a temp directory", () => {
     const home = process.env.HOME;
+    const userProfile = process.env.USERPROFILE;
 
     expect(home).toBeDefined();
     expect(home).toContain(tmpdir());
     expect(home).toContain(TEMP_HOME_PREFIX);
+    expect(userProfile).toBe(home);
   });
 
   it("homedir() resolves to the temp HOME", () => {