[dependencies] Evaluate replacing JSONSchema.swift with DynamicJSON

[DandyLyons]

Problem / outcome

After #124 introduces DynamicJSON for RFC 9535 JSONPath, assess and, if parity is demonstrated, migrate md-utils JSON Schema validation from JSONSchema.swift to DynamicJSON. The goal is to consolidate the portable JSON value/query/schema stack and remove the JSONSchema.swift dependency and its WASI compatibility patch only after all existing behavior is preserved.

This issue is justified by an initial source and compatibility assessment: DynamicJSON includes a Draft 2020-12 validator, external resource providers and registries, anchors and dynamic anchors, located validation results, annotations/defaults, no package dependencies, and current Swift Package Index builds for Apple platforms, Linux, and WebAssembly.

In scope

• Build a narrow md-utils schema-validation adapter around DynamicJSON.
• Verify parity for MarkdownTypeChecker, MarkdownRuleChecker, CLI configuration validation, fm-var fixture validation, and OKF tests.
• Preserve the existing explicit schema-resource model: no implicit filesystem or network retrieval.
• Preserve external reference graph handling, canonical identifiers, nested identifiers, anchors, dynamic references, missing-resource errors, cycle rejection, and conflicting identifier rejection.
• Preserve stable instance locations and required-property diagnostics/fix-its, or document and test any intentional diagnostic changes.
• Isolate DynamicJSON's mutable, documented non-thread-safe registry so public md-utils types remain concurrency-safe.
• Compare DynamicJSON's vendored JSON Schema conformance suite and ignored cases with the behavior md-utils relies on.
• Verify Apple, native Linux, and WebAssembly builds before changing dependencies.
• Remove JSONSchema.swift and the version-specific WASI patch only after the migration is complete and verified.

Acceptance criteria

• Existing JSON Schema tests pass through the DynamicJSON adapter, including Draft 2020-12 const, contains, allOf, required-property, and nested external-reference coverage.
• External resources are supplied only by the caller; validation performs no filesystem or network I/O.
• Missing resources, cycles, conflicting identifiers, anchors, and dynamic references have deterministic tested behavior.
• Validation diagnostics retain useful instance locations and required-property fix-it behavior.
• Registry and validator use are safe under Swift 6 strict concurrency without exposing shared mutable non-Sendable state.
• MarkdownUtilitiesCore builds and representative validation runs on Apple platforms, native Linux, and WebAssembly.
• JSONSchema.swift is removed from Package.swift only after parity is demonstrated, and the obsolete WASI compatibility patch and documentation are removed in the same change.

Explicitly out of scope

• Removing jmespath.swift. DynamicJSON implements JSONPath, not JMESPath; md-utils currently exposes JMESPath as user-facing syntax in fm search, fm unique, and rule configuration.
• Implementing JSONPath in this issue; that remains [fm-var] Implement RFC 9535 JSONPath querying #124.
• Hand-rolling a JSON Schema implementation.

Dependency

• Follow up after or alongside [fm-var] Implement RFC 9535 JSONPath querying #124 once DynamicJSON is present in the dependency graph and its supported version is pinned by normal SwiftPM resolution.