fix: drop universal_io so the package is WASM compatible

[Eimen2018]

Summary

Makes stacked WASM compatible on pub.dev. Refs #1105.

Problem

pub.dev marks stacked 3.5.0 as not WASM compatible, and stacked_services inherits that because it imports stacked. The reason pub.dev gives:

package:stacked/stacked.dart → router_utils.dart → package:universal_io/io.dart → ... → internet_address.dart → dart:io

universal_io was only used for one check, Platform.isIOS || Platform.isMacOS in buildAdaptivePageRoute, and that check already sits behind a kIsWeb guard. The latest universal_io is itself not WASM ready, so upgrading it does not help.

Change

• router_utils.dart: use defaultTargetPlatform from flutter/foundation instead of Platform.
• pubspec.yaml: remove universal_io.

Behaviour is the same on every platform. A side benefit is that the check now respects debugDefaultTargetPlatformOverride in tests.

Verification

• pana (the analyser pub.dev uses) on this branch: is:wasm-ready, platform support 20/20. On master it is not WASM ready.
• Built a test app with flutter build web --wasm against this branch and ran it in a browser. Confirmed it ran the .wasm module with no JS fallback, buildAdaptivePageRoute worked, and the console was clean.
• flutter test: 93 passed. dart analyze: no issues in the changed file.

🤖 Generated with Claude Code

[github-actions]

LCOV of commit bf641d9 during Test Package #156

📊 Summary Coverage

Metric	Coverage	Details
📈 Lines	🔴 7.6%	279 / 3689 lines
🔧 Functions	⚪ N/A	no data found
🌿 Branches	⚪ N/A	no data found

📁 Files Changed Coverage

File	Lines	Functions	Branches
lib/src/code_generation/router_annotation/extended_navigator.dart	⚪ – (0)	⚪ – (0)	⚪ – (0)
lib/src/code_generation/router_annotation/router_utils.dart	⚪ – (0)	⚪ – (0)	⚪ – (0)

🟢 ≥ 90% · 🟡 ≥ 70% · 🔴 below 70% · ⚪ no data
Full coverage report

[ferrarafer]

LGTM