Define pinned offline diagram validation capabilities for shared architecture checks

[szmyty]

Outcome

Define a reviewed, immutable offline validation capability for diagram sources used by the shared architecture-validation pipeline, preserving the diagram projects' authority over their languages and file formats.

Observed gap

Relay #99 checkpoint 3 can inventory bounded Mermaid, PlantUML and Excalidraw sources and emit safe metadata. The existing profile still declares diagram semantics planned: none of its three reviewed source inputs supplies a pinned format validator. Discovery completeness therefore cannot establish diagram validity.

This is a validator capability gap, separate from EgoLint #73's ADR policy-pin compatibility repair. No existing EgoLint diagram issue was found during the 2026-09-26 checkpoint review.

Scope and acceptance

• Evaluate supported offline entry points from the format owners; document supported syntax/file versions and unavailable capabilities.
• Publish a versioned capability contract with immutable source/artifact pins, dependency hashes, diagnostics and exact failure semantics for Relay to consume.
• Keep diagram language/schema semantics with the upstream format owners; Relay validates only orchestration metadata.
• Prove no network access, no consumer code/config execution, no host-file reads or writes, and bounded CPU, memory, input and report size.
• Cover embedded Markdown blocks as well as standalone source; define the handling of includes, external assets, callbacks, scripts, browser dependencies, embedded image data and unsupported versions.
• Preserve unknown, invalid, unavailable, empty and truncated states separately; never infer validity from successful discovery or rendering alone.
• Add positive, malformed and hostile fixtures with stable rule IDs, source regions, remediation and sanitized machine-readable results.
• Supply the reviewed immutable input needed for a separate Relay profile/adaptor upgrade. If a format cannot meet the offline trust boundary, keep it explicitly unavailable and record the reason.

Primary interfaces to review

• Mermaid project CLI: https://github.com/mermaid-js/mermaid-cli
• PlantUML command-line interface: https://plantuml.com/command-line
• Excalidraw source format: https://docs.excalidraw.com/docs/codebase/json-schema

These references identify upstream interfaces, not approved versions or authorization to execute them in consumers.

Related

• Deliver #5 through an ordered architecture-validation execution plan relay#99 — ordered shared architecture-validation implementation
• Validate architecture and repository structure in CI relay#5 — parent acceptance
• [Capability 1] Roll out and reconcile the ADR baseline and Decisions views across the fleet pace#5 — ADR-first fleet campaign

Release publication, required enforcement and fleet rollout remain separately reviewed.