refactor: simplify review fixes for WebM PR

- Use static import for copyFileSync (was unnecessary dynamic import)
- Shallow-copy config before mutating forceScreenshot (prevents
  caller-provided config from being permanently modified)
- Consolidate isWebm/isWebmRender/outputFormat into single early
  declaration in renderOrchestrator
- Fix debug output extension for WebM (was hardcoded .mp4)
- Log unexpected audio extraction errors instead of silently swallowing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/engine/src/services/chunkEncoder.ts

@@ -6,7 +6,7 @@
  */
 
 import { spawn } from "child_process";
-import { existsSync, mkdirSync, readdirSync, statSync, writeFileSync } from "fs";
+import { copyFileSync, existsSync, mkdirSync, readdirSync, statSync, writeFileSync } from "fs";
 import { join, dirname } from "path";
 import { DEFAULT_CONFIG, type EngineConfig } from "../config.js";
 import { type GpuEncoder, getCachedGpuEncoder, getGpuEncoderName } from "../utils/gpuEncoder.js";
@@ -414,8 +414,6 @@ export async function applyFaststart(
 ): Promise<MuxResult> {
   // faststart is MP4-only (moves moov atom to file start for streaming)
   if (outputPath.endsWith(".webm")) {
-    // For WebM, just copy the file as-is
-    const { copyFileSync } = await import("fs");
     if (inputPath !== outputPath) copyFileSync(inputPath, outputPath);
     return { success: true, outputPath, durationMs: 0 };
   }