fix: fix sourcemap formatter tests for ESM and cross-platform paths

- Add __filename polyfill for ESM modules using import.meta.url
- Fix FILE_URL_REGEX to preserve leading slash in file:/// URLs
- Ensure absolute paths remain absolute after normalization

The tests were failing in CI because:
1. __filename is not available in ESM modules
2. The file:// URL regex was removing all slashes including the path separator

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: johnlindquist

src/core/sourcemap-formatter.test.ts

@@ -1,6 +1,9 @@
 import ava from 'ava'
 import { SourcemapErrorFormatter } from './sourcemap-formatter.js'
 import os from 'os'
+import { fileURLToPath } from 'url'
+
+const __filename = fileURLToPath(import.meta.url)
 
 ava('formatError should parse basic error stack', (t) => {
   const error = new Error('Test error')

src/core/sourcemap-formatter.ts

@@ -23,7 +23,7 @@ export interface FormattedError {
 export class SourcemapErrorFormatter {
   private static readonly STACK_FRAME_REGEX = /^\s*at\s+(?:(.+?)\s+\()?(.+?):(\d+):(\d+)\)?$/
   private static readonly WINDOWS_PATH_REGEX = /^([a-zA-Z]:\\|\\\\)/
-  private static readonly FILE_URL_REGEX = /^file:\/\/\/?/
+  private static readonly FILE_URL_REGEX = /^file:\/\//
 
   /**
    * Formats an error with enhanced stack trace information
@@ -63,9 +63,15 @@ export class SourcemapErrorFormatter {
           file = file.substring(1)
         }
 
-        // Normalize path for current platform
+        // Ensure absolute paths remain absolute after normalization
+        const isAbsolutePath = file.startsWith('/') || this.WINDOWS_PATH_REGEX.test(file)
         file = normalize(file)
 
+        // On Unix, normalize may strip leading slash, restore it if needed
+        if (isAbsolutePath && !file.startsWith('/') && os.platform() !== 'win32') {
+          file = '/' + file
+        }
+        
         frames.push({
           file,
           line: parseInt(lineStr, 10),
@@ -155,10 +161,16 @@ export class SourcemapErrorFormatter {
       }
 
       // Resolve relative paths
-      const resolvedPath = isAbsolute(cleanPath) 
+      const isAbsolutePath = cleanPath.startsWith('/') || this.WINDOWS_PATH_REGEX.test(cleanPath)
+      let resolvedPath = isAbsolute(cleanPath) 
         ? normalize(cleanPath)
         : resolve(basePath || process.cwd(), cleanPath)
 
+      // On Unix, normalize may strip leading slash, restore it if needed  
+      if (isAbsolutePath && !resolvedPath.startsWith('/') && os.platform() !== 'win32') {
+        resolvedPath = '/' + resolvedPath
+      }
+      
       // Check if file exists
       if (existsSync(resolvedPath)) {
         return resolvedPath