[gmatossian]

Adds the five owner-approved Spring and validation references, with Spring reachable under Java and the shared validation guide under System Design. Bean Validation has both contextual paths. The content includes the reviewed ecosystem/entry-path diagrams, compact tables, controller/error examples, and an Actuator health check under running the app.

Adds only the network and validation icon keys/assets needed by these topics, plus license and asset coverage. Documents optional authoring-only sources.md files so provenance stays available without occupying reader-facing content. Unrelated pending content and the broader icon migration are excluded.

Closes #123.

Validation and review

• npm run check passed on Node 24.20.0: formatting, lint, tooling types, 57 app tests, 144 tooling tests, production build, and 38 browser tests.
• Reviewed the complete final staged diff before push; no blocking findings. Confirmed the five topic bodies/assets match the owner-reviewed drafts and all remain accepted.
• Manually checked the isolated production build at desktop and 390px widths, including both Bean Validation paths, diagrams, tables, and setup/Actuator/error content. Authoring-source text is absent from generated and bundled content.
• Earlier example verification used an Initializr-generated Boot 4.1.2 Maven/Java 21 project: packaged JAR and Maven runs returned POST 201 with Location and GET 200 with matching product JSON. A fresh app without controllers returned Actuator health HTTP 200 with status UP after adding the shown dependency.
• Existing non-fatal initial-bundle budget warning remains; no dependency or build-policy changes.

AI-assisted preparation, integration, verification, and deliberate self-review by Codex. The owner reviewed/accepted the content and explicitly authorized conditional merge after successful Actions; no independent reviewer was selected for this bounded content integration.