fix(build/externals): propagate bundle failures instead of silent stubs

`bundlePackage` caught every error, wrote a `throw new Error(...)` stub
to the output path, and returned `undefined`. Consequences:
- The orchestrator's `try { await bundlePackage(...) } catch` blocks
  for optional packages were unreachable — the outer catch never fired.
- The build completed "successfully" even when bundles failed. The
  "Failed to bundle X" log line scrolled past amid the rest of the
  output, and downstream consumers didn't learn anything was wrong
  until they `require()`'d the package at runtime and hit the stub's
  `throw new Error('Failed to bundle X')`.

Rethrows the error. Optional-package try/catch now works as intended,
and non-optional failures bubble up to `buildExternals` → `main.mts` so
the build exits non-zero.

Author: jdalton

scripts/build-externals/bundler.mts

@@ -114,12 +114,10 @@ ${contentWithoutStrict}`
     if (!quiet) {
       logger.log(`    ✗ Failed to bundle ${packageName}: ${error.message}`)
     }
-    // Create error stub.
-    const stubContent = `'use strict'
-
-// Failed to bundle ${packageName}: ${error.message}
-throw new Error('Failed to bundle ${packageName}')
-`
-    await fs.writeFile(outputPath, stubContent)
+    // Propagate the failure. The orchestrator wraps optional packages in
+    // try/catch and logs a "Skipping optional package" message; required
+    // packages bubble up so the build exits non-zero instead of silently
+    // shipping throw-stubs that only fail later, at consumer runtime.
+    throw error
   }
 }