Add inspector diagnostics to reports

[JuanTorchia]

Summary

• Add an inspector diagnostics model with source, severity, message, and optional path.
• Carry diagnostics through ProjectMetadata and preserve them when the CLI combines build metadata with source patterns.
• Render diagnostics in a dedicated Markdown report section.
• Emit a Gradle version catalog warning when gradle/libs.versions.toml cannot be parsed while preserving visible build-file inspection.

Validation

• RED: mvn -pl analyzer-core,build-inspectors -am '-Dtest=MarkdownReportRendererTest,GradleProjectInspectorTest' '-Dsurefire.failIfNoSpecifiedTests=false' test '-Dmaven.compiler.release=22' first failed because the diagnostics model did not exist, then failed because malformed catalog diagnostics were not emitted.
• mvn -pl analyzer-core,build-inspectors -am '-Dtest=MarkdownReportRendererTest,GradleProjectInspectorTest' '-Dsurefire.failIfNoSpecifiedTests=false' test '-Dmaven.compiler.release=22'
• mvn clean test '-Dmaven.compiler.release=22'
• python -B scripts/check-local-markdown-links.py
• git diff --check

Closes #25.