FEP-fe34 origin-based security model

Implements comprehensive origin-based security checks following FEP-fe34
specification to prevent content spoofing attacks and maintain secure
federation practices.

Key changes:

- Add crossOrigin option to property accessors and lookupObject() with
  three modes: "ignore" (default), "throw", and "trust"
- Implement trust tracking system for embedded objects using trust sets
- Add origin validation for object @id vs document URL in lookupObject()
- Add origin validation for property objects vs their parent object
- Update documentation with security model explanations and examples
- Add comprehensive tests for all cross-origin scenarios

This replaces the previous FEP-c7d3 ownership model with the more robust
origin-based approach, ensuring objects and their properties respect
origin boundaries to prevent malicious content spoofing.

See also http://w3id.org/fep/fe34

Fixes https://github.com/fedify-dev/fedify/issues/440



Co-Authored-By: Claude <noreply@anthropic.com>