Add YAML schema adapter

[eviltester]

Summary

Adds a V1 YAML ER schema adapter for Thingifier and a neutral schema definition layer that can be reused by other text/model adapters later.

What Changed

• Added a new thingifier-yaml Maven module with SnakeYAML scoped to the adapter module.
• Added YAML-agnostic schema definition specs in thingifier for entities, fields, validations, relationships, cardinality, optionality, and validation reporting.
• Added ThingifierModelAssembler to validate neutral specs and build a normal Thingifier through the existing Java model API.
• Added ThingifierModelExporter to export Java-built runtime schemas back to canonical neutral schema definitions.
• Added ThingifierYamlLoader and ThingifierYamlExporter public adapter APIs.
• Added YAML resource fixtures for valid and invalid ER models.
• Added static guardrails to keep SnakeYAML/YAML DTOs out of thingifier and ercoremodel.
• Added read-only core metadata getters needed for schema export without changing validation behavior.

Validation

• mvn -pl thingifier-yaml -am test
• mvn -pl thingifier-yaml -am -DskipTests checkstyle:check@project-fqn-check
• mvn spotless:check
• mvn -pl ercoremodel,thingifier,thingifier-yaml -am pmd:check checkstyle:check
• mvn -pl thingifier,thingifier-yaml,challenger -am verify
• JaCoCo for thingifier-yaml: 96.89% line coverage, 96.42% instruction coverage, 86.90% branch coverage

Closes #42

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