test(lib): cover request-header building + log ring buffer (4→20 tests)

Extract the auth + directory-header encoding from sdk.ts into a pure, dep-free
headers.ts (sdk.ts now delegates — behavior unchanged) so the connection-critical
logic is unit-testable without expo/fetch. Add 8 header tests (ASCII passthrough,
non-ASCII/CJK percent-encoding stays header-safe, Basic auth, empty-dir handling)
and 8 logbuffer tests (serialization incl. circular-ref fallback, 200-entry ring
cap, copy semantics, formatting). typecheck clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/sdk.ts

@@ -3,6 +3,7 @@
 // but works in React Native environment
 // expo/fetch provides WinterCG-compliant fetch with ReadableStream support for SSE
 import { fetch as expoFetch } from "expo/fetch"
+import { buildRequestHeaders } from "./headers"
 
 export interface ClientConfig {
   baseUrl: string
@@ -154,21 +155,7 @@ export interface HealthResponse {
 const REQUEST_TIMEOUT_MS = 30_000
 
 function createHeaders(config: ClientConfig): HeadersInit {
-  const headers: HeadersInit = {
-    "Content-Type": "application/json",
-  }
-
-  if (config.directory) {
-    const encoded = /[^\x00-\x7F]/.test(config.directory) ? encodeURIComponent(config.directory) : config.directory
-    headers["x-opencode-directory"] = encoded
-  }
-
-  if (config.auth) {
-    const credentials = btoa(`${config.auth.username}:${config.auth.password}`)
-    headers["Authorization"] = `Basic ${credentials}`
-  }
-
-  return headers
+  return buildRequestHeaders(config)
 }
 
 async function request<T>(config: ClientConfig, path: string, options: RequestInit = {}): Promise<T> {