feat: harden diagnose and lint workflows

[sjquant]

Why

Make diagnostics reliable for automated workflows and consistent across Canvas and Deck specs.

Changes

• normalize invalid JSON validation errors and provide structured JSON error responses
• add quickthumb diagnose as a lint alias with --fail-on and repeatable --ignore controls
• support Deck JSON in lint output while preserving slide and layer diagnostic context
• retain valid edge-crowding findings for visible covering layers and suppress duplicate text-wrap warnings
• update diagnostic API/CLI documentation and add black-box regression coverage