feat: add published JSON schema CLI

[sjquant]

Why

Expose a deterministic JSON Schema for quickthumb JSON specs so agents, editors, and constrained-generation workflows can use the same model contract as the renderer.

Changes

• Added quickthumb schema with stdout output and optional --output file writing.
• Added a public canvas_json_schema() helper backed by Pydantic spec models, including built-in layer discriminators, top-level theme, explicit canvas sizing, and supported platform presets.
• Tightened generated schema metadata for colors, opacity, position arrays, positive percentage fields, enum-like fields, and tuple-style align inputs without widening stored runtime model types.
• Documented schema usage and constrained-generation guidance in the JSON workflow docs and README.

Validation

• uv run --locked ty check quickthumb --output-format concise
• uv run --locked ruff check quickthumb tests
• uv run --locked ruff format --check quickthumb tests
• uv run --locked pytest tests/test_cli.py tests/test_canvas.py --no-cov