fix: reliable, self-healing AI feature-bundle installs (#472)

Make on-demand AI feature-bundle installs reliable and self-healing, closing
the failure modes behind most "some tool doesn't work" reports.

Multi-bundle installs: tools needing more than one bundle (Passport Photo,
Enhance Faces) install every required bundle from one action and stay
not-installed until all are present. Verified across all 19 AI tools.

Downloads: self-heal the accelerated Hugging Face (Xet) client so an upgraded
venv no longer silently falls back to slow urllib; restart instead of
corrupting a resumed partial when a proxy ignores Range and returns 200;
verify the completed size; fail fast on disk-full and HTTP 4xx; retry
transient errors five times; add hf_transfer fallback and document Xet egress.

Install integrity: crash-atomic venv writes so a killed or out-of-space
install can no longer tear the shared venv and break other tools; a boot
breadcrumb reseeds a torn venv to a clean state automatically; a post-install
smoke import test refuses to record a bundle whose libraries cannot load; an
install watchdog stops a wedged installer that would otherwise hold the venv
writer lock forever.

Adds unit and end-to-end tests for every failure mode above.

tests/unit/ai/gpu-detection.test.ts

@@ -67,6 +67,7 @@ describe("buildMinimalEnv - env passthrough", () => {
 
   afterEach(() => {
     vi.restoreAllMocks();
+    delete process.env.DATA_DIR;
     delete process.env.SNAPOTTER_GPU;
     delete process.env.MODELS_PATH;
     delete process.env.MODELS_DIR;
@@ -110,6 +111,24 @@ describe("buildMinimalEnv - env passthrough", () => {
     expect(env?.MODELS_PATH).toBe("/data/ai/models");
   });
 
+  it("sets matching native DATA_DIR and MODELS_PATH defaults", async () => {
+    delete process.env.DATA_DIR;
+    delete process.env.MODELS_PATH;
+
+    const mock = createMockProcess();
+    vi.mocked(spawn).mockReturnValue(mock.process);
+
+    const promise = runPythonWithProgress("test.py", []);
+    mock.stdout.emit("data", Buffer.from('{"ok": true}\n'));
+    mock.emitEvent("close", 0, null);
+    await promise;
+
+    const env = getSpawnEnv();
+    expect(env).toBeDefined();
+    expect(env?.DATA_DIR).toBe("./data");
+    expect(env?.MODELS_PATH).toBe("./data/ai/models");
+  });
+
   it("does not pass MODELS_DIR (removed dead entry)", async () => {
     process.env.MODELS_DIR = "/some/path";