feat(sdk): add PostHog usage tracking (#1999)

* feat(sdk): add PostHog usage tracking

Track SDK method calls (localize*, recognizeLocale) with start/success/error
events via PostHog. Identity is resolved through /whoami endpoint with
fallback to hashed API key. Respects DO_NOT_TRACK=1 for opt-out.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* chore: add changeset for SDK PostHog tracking

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(sdk): correct SDK_PACKAGE name and Content-Type header

Fix SDK_PACKAGE constant to match actual package name "@lingo.dev/_sdk"
and fix malformed "ContentType" header to proper "Content-Type" in
whoami request headers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: cherkanovart

.changeset/sdk-posthog-tracking.md

@@ -0,0 +1,5 @@
+---
+"@lingo.dev/_sdk": minor
+---
+
+Add PostHog usage tracking for SDK method calls

packages/sdk/package.json

@@ -33,6 +33,7 @@
     "@lingo.dev/_spec": "workspace:*",
     "@paralleldrive/cuid2": "2.2.2",
     "jsdom": "25.0.1",
+    "posthog-node": "5.14.0",
     "zod": "4.1.12"
   },
   "devDependencies": {

packages/sdk/src/index.spec.ts

@@ -1,4 +1,7 @@
 import { describe, it, expect, vi } from "vitest";
+
+vi.mock("./utils/observability");
+
 import { LingoDotDevEngine } from "./index";
 
 describe("ReplexicaEngine", () => {
@@ -95,8 +98,8 @@ describe("ReplexicaEngine", () => {
   describe("localizeStringArray", () => {
     it("should localize an array of strings and maintain order", async () => {
       const engine = new LingoDotDevEngine({ apiKey: "test" });
-      const mockLocalizeObject = vi.spyOn(engine, "localizeObject");
-      mockLocalizeObject.mockImplementation(async (obj: any) => {
+      const mockLocalizeRaw = vi.spyOn(engine as any, "_localizeRaw");
+      mockLocalizeRaw.mockImplementation(async (obj: any) => {
         // Simulate translation by adding 'ES:' prefix to all string values
         return Object.fromEntries(
           Object.entries(obj).map(([key, value]) => [key, `ES:${value}`]),
@@ -110,8 +113,8 @@ describe("ReplexicaEngine", () => {
         targetLocale: "es",
       });
 
-      // Verify the mapped object was passed to localizeObject
-      expect(mockLocalizeObject).toHaveBeenCalledWith(
+      // Verify the mapped object was passed to _localizeRaw
+      expect(mockLocalizeRaw).toHaveBeenCalledWith(
         {
           item_0: "Hello",
           item_1: "Goodbye",
@@ -130,15 +133,15 @@ describe("ReplexicaEngine", () => {
 
     it("should handle empty array", async () => {
       const engine = new LingoDotDevEngine({ apiKey: "test" });
-      const mockLocalizeObject = vi.spyOn(engine, "localizeObject");
-      mockLocalizeObject.mockImplementation(async () => ({}));
+      const mockLocalizeRaw = vi.spyOn(engine as any, "_localizeRaw");
+      mockLocalizeRaw.mockImplementation(async () => ({}));
 
       const result = await engine.localizeStringArray([], {
         sourceLocale: "en",
         targetLocale: "es",
       });
 
-      expect(mockLocalizeObject).toHaveBeenCalledWith(
+      expect(mockLocalizeRaw).toHaveBeenCalledWith(
         {},
         {
           sourceLocale: "en",