Fix lexical lifetime of using declarations

[DavidObando]

Summary

• bind top-level statements and block-expression prefixes as lexical statement lists
• keep using/defer cleanup around trailing expressions, awaits, returns, and exceptions
• add runtime ordering and async MoveNext IL coverage for closures and nested resources

Validation

• related using/async tests: 25 passed
• Core.Tests: 6,691 passed, 1 skipped
• Compiler.Tests: 3,407 passed
• patched migrated Oahu CLI: doctor --skip-network reports all checks passed

Deferred work

• None.

Fixes #2753