test: add comprehensive unit tests for all appenders and logger

Tests added:
- log-service.test.ts: hierarchical logger creation and threshold propagation
- logger.test.ts: threshold inheritance, message propagation, falsy threshold
  bug coverage (LogLevel.ALL = 0), deferred messages, Error handling
- log-message.test.ts: parseLogLevel parsing, flattenMessage object/string modes
- console-log-appender.test.ts: threshold, formatter, all-level routing
- buffer-log-appender.test.ts: buffer append, threshold filtering, clearBuffer
- api-log-appender.test.ts: configuration defaults, threshold, auth token

Also fixes src/index.ts to export BufferLogAppender which was previously
missing from the barrel export.

test/log-service.test.ts

@@ -1,5 +1,5 @@
 import { describe, test, expect } from "bun:test";
-import { LogService } from "../src/log-service";
+import { LogService, LogLevel } from "../src";
 
 describe("LogService", () => {
   test("creates a root logger on construction", () => {
@@ -8,4 +8,47 @@ describe("LogService", () => {
     expect(svc.ROOT_LOGGER).toBeDefined();
     expect(svc.ROOT_LOGGER.name).toBe("ROOT");
   });
+
+  test("root logger defaults to ALL threshold", () => {
+    const svc = new LogService();
+
+    expect(svc.ROOT_LOGGER.threshold).toBe(LogLevel.ALL);
+  });
+
+  test("getLogger returns the same instance for the same name", () => {
+    const svc = new LogService();
+
+    const a = svc.getLogger("foo");
+    const b = svc.getLogger("foo");
+
+    expect(a).toBe(b);
+  });
+
+  test("getLogger creates hierarchical loggers", () => {
+    const svc = new LogService();
+
+    const foo = svc.getLogger("foo");
+    const fooBar = svc.getLogger("foo.bar");
+    const fooBarBaz = svc.getLogger("foo.bar.baz");
+
+    // foo.bar should be a child of foo
+    // We can verify by checking threshold propagation
+    foo.appenders = [{
+      threshold: LogLevel.ALL,
+      messages: [] as any[],
+      appendMessage(m: any) { this.messages!.push(m); },
+    }];
+
+    fooBar.info("propagate");
+
+    // The foo appender should receive the message from fooBar
+    expect(foo.appenders[0].messages.length).toBe(1);
+  });
+
+  test("getLogger allows setting threshold", () => {
+    const svc = new LogService();
+
+    const logger = svc.getLogger("with-threshold", LogLevel.ERROR);
+    expect(logger.threshold).toBe(LogLevel.ERROR);
+  });
 });