fix(sensors_plus): isolate iOS stream handlers per engine

[FeliksLv01]

Description

Fixes the iOS lifecycle crash and cross-engine stream interference described in #3955.

The plugin now publishes an engine-owned instance and implements detachFromEngine(for:). Event channels and stream handlers are instance-owned, while each handler owns its CoreMotion object and event sink. Detaching one FlutterEngine therefore only stops that engine's streams; queued callbacks re-check the handler state before sending an event.

Related Issues

• Fixes [Bug]: [sensors_plus] Termination crash fix from 1.3.3 is unreachable: plugin instance never published, detach hook has wrong selector #3955

Checklist

• I read the Contributor Guide and followed the required package-specific branch and PR workflow.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer does not report any problems on this PR.

Validation

• flutter build ios --no-codesign --simulator --debug --target=lib/main.dart --dart-define=CI=true using Flutter 3.47.0.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.