debug(sea-builder): add diagnostic output for postject resolution

Add detailed error logging to diagnose why postject is not being found
on Windows CI runners despite being in devDependencies. This will help
identify whether it's a PATH issue, installation issue, or pnpm exec
resolution problem.

Logs include:
- Error message from spawn
- Platform and CWD
- Existence checks for postject in various node_modules paths

Author: jdalton

packages/node-sea-builder/scripts/build.mjs

@@ -346,7 +346,26 @@ async function injectSeaBlob(nodeBinary, blobPath, outputPath, fuseSentinel) {
     await spawn('pnpm', ['exec', 'postject', '--help'], {
       stdio: 'ignore',
     })
-  } catch {
+  } catch (e) {
+    logger.error('Failed to execute postject check')
+    logger.error(`Error: ${e.message}`)
+    logger.error(`Platform: ${process.platform}`)
+    logger.error(`CWD: ${process.cwd()}`)
+
+    // Log node_modules structure for debugging.
+    try {
+      const { existsSync } = await import('node:fs')
+      const postjectPaths = [
+        'node_modules/.bin/postject',
+        '../../node_modules/.bin/postject',
+        'node_modules/postject',
+        '../../node_modules/postject',
+      ]
+      for (const p of postjectPaths) {
+        logger.error(`  ${p}: ${existsSync(p) ? 'exists' : 'not found'}`)
+      }
+    } catch {}
+
     throw new Error(
       'postject is required to inject the SEA blob into the Node.js binary.\n' +
         'Please install it: pnpm add -D postject',