Implement canonical ordered collections (#415)

[szmyty]

Summary

• Accept one explicit, ordered, homogeneous renderflow/v2 collection in canonical planning and execution, with required member ID, root-relative path, format, media type, and SHA-256; optional geometry enters plan identity.
• Freeze member intake and order in the execution plan, recheck paths and bytes at execution, import each member into the durable artifact store, and retain ordered source/aggregate lineage in run evidence. Bind transform registry configuration and the collection cache/checkpoint context to the exact plan.
• Add a two-page synthetic aggregation fixture, a generated 44-member recipe, CLI/SDK/refusal tests, schema, generated reference, user guide, and roadmap handoff.

Local validation

• cargo test --workspace --locked --quiet: 1,187 passed, 11 ignored, 0 failed.
• cargo clippy --workspace --all-targets --locked -- -D warnings and cargo fmt --all --check: passed with Rust 1.98.
• cargo package --package renderflow --allow-dirty --locked: packaged and verified.
• mkdocs build --strict: passed. Schema and generated spec reference regenerated idempotently.
• Only synthetic fixture files were executed. Hosted CI was not polled.

Boundary

This adds the collection source contract for #416 and #417; it does not implement PDF/EPUB exporters or apply to personal media. Existing single-source v1/v2 execution remains covered by the workspace suite. The development changes are unreleased.

Closes #415