Add Structured Diagnostic JSON Output

[sjquant]

Why

Make quickthumb diagnostics machine-readable so lint output can feed automated repair loops while keeping the existing human-readable lint output intact.

Changes

• Add optional structured diagnostic fields for layer identity, bbox, related layers, measured values, and suggestions.
• Populate structured fields for current off-canvas, tiny text, text overflow, and low-contrast diagnostics from measured layer data.
• Make off-canvas suggestions account for oversized layers and aligned layer position semantics.
• Add quickthumb lint --format json with summary counts and structured diagnostics.
• Document the structured diagnostic fields and JSON lint payload.
• Cover structured diagnostics, JSON lint output, invalid lint formats, and suggestion edge cases with integration tests.