Respect task overrides and reuse application state

[OskarEichler]

Summary

• honor explicit environment, output, assets, and manifest assignments on Rails-aware asset tasks
• memoize the application environment and manifest instead of rebuilding them on repeated reads
• preserve superclass behavior when no Rails application is attached

Verification

Cumulative unmodified suites pass 151 tests / 796 assertions in three Ruby/Rails/Sprockets configurations; focused task models cover explicit overrides, repeated reads and application/no-application paths. Syntax, package and backend integration checks pass. No test files were changed.