Add inspectable real-artifact gallery for three synthetic inputs (#433)

[szmyty]

Summary

• Add the first bounded Build a comprehensive adversarial rendering fixture corpus #412 gallery checkpoint with one original synthetic Markdown input and two original synthetic RGB PNG pages.
• Run the public Renderflow CLI with Pandoc and img2pdf to produce inspectable HTML and a two-page print PDF in two independent clean builds. Keep generated files and run manifests under a caller-selected output directory.
• Check the reviewed HTML oracle, PDF page/box/image-stream oracle, source and output digests, ordered lineage, provider evidence, validation, and absence of unexpected published files. Retain typed refusals for changed source bytes, invalid provider output, and a truncated PDF.
• Extend the versioned corpus and schema, document the local runner, and update the roadmap handoff.

Local validation

• cargo test --workspace --locked --quiet — passed (the real-provider gallery remains explicitly ignored in the default tier).
• scripts/run-artifact-gallery.sh --output-dir /workspace/scratch/be7f1f8a088f/gallery-output-433-final-2 --img2pdf /workspace/scratch/be7f1f8a088f/renderflow-pdf-deps/bin/img2pdf — passed with Pandoc 3.1.3 and img2pdf 0.6.3; two artifact cases, two clean builds each, three refusal checks.
• cargo clippy --workspace --all-targets --locked -- -D warnings, cargo fmt --all --check, and strict MkDocs build — passed.
• Synthetic inputs only. Hosted CI, native macOS, and physical print validation were not run.

Run this on your machine from the repository root with scripts/run-artifact-gallery.sh --output-dir "$PWD/gallery-output"; see docs/artifact-gallery.md for provider requirements and retained evidence.

Closes #433
Refs #412

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