Add canvas inspection report

[sjquant]

Why

Expose a deterministic canvas.inspect() report so agents can read measured layout state without rendering output or invoking diagnostics.

Changes

• Add public inspection report models for canvas, layer, bbox, and text layout metadata.
• Add canvas.inspect() backed by measure_layers(canvas), including layer IDs, order/z-order, visibility, bboxes, group children, and text wrapped lines/effective font sizes.
• Keep inspect-only text layout work at the inspection boundary so diagnostics and plain measurement stay lightweight.
• Add black-box inspection tests for text, rich auto-scaled text, shapes, images, SVGs, groups, custom layers, invisible layers, stable order/IDs, and inspect-only metadata behavior.

Validation: uv run pytest tests/test_inspection.py tests/test_diagnostics.py; uv run ruff check quickthumb tests/test_inspection.py; touched-file formatter check passed.