Simplify enum deduplication logic in JtdPropertyTest

[simbo1905]

Issue #99: Enforce RFC 8927 semantics for {} schema and remove AJV compatibility

Problem

Current codebase contains legacy AJV/JSON Schema compatibility where {} could mean "accept anything".
This contradicts RFC 8927: {} always means an object with no properties allowed
(properties = {}, optionalProperties = {}, additionalProperties = false).

This dual behavior causes complexity and test contradictions.

Goals

1. Rip out all AJV-style "compatibility mode" handling (fromRef, EmptySchema fallback).
2. Make {} always RFC 8927 strict (PropertiesSchema with no properties, no additionalProperties).
3. Add an INFO log warning whenever {} is compiled to help developers spot semantic differences from AJV/JSON Schema.
4. Rewrite AJV-derived tests to expect failures or exceptions under RFC mode.
5. Update README.md and AGENTS.md to document the difference.

Implementation Notes

• Follow Markdown-Driven-Development: update documentation before code
• Add INFO-level logging when {} schema is compiled
• Ensure all tests validate strict RFC 8927 behavior
• Update user-facing documentation to explain the semantic difference

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