Resolve nested includes relative to their template

[OskarEichler]

Summary

• resolve each nested relative include from the directory of the template that contains it
• restore the previous include context after successful or failed recursive compilation
• document nested relative-path behavior

The current filter always uses the root template's file option, so a template in nested/parent.slim cannot include its sibling with include sibling when include_dirs is relative.

Verification

• focused four-level nested/sibling include and exception-cleanup model on Ruby 4.0.6 and 3.2.11
• existing core/plugin suite: 415 runs, 650 assertions on each Ruby
• Rails 8.1.3.1: 15 runs, 36 assertions on each Ruby
• Sinatra 4.2.1: 10 runs, 21 assertions on each Ruby
• syntax, package surface and diff checks pass

No test files are changed.