Advance v0.20.0 renderer and Formation closure

[snkmcb]

Summary

• bind managed renderer build/test reports to completion records with session IDs and SHA-256 digests
• persist structured viewport launch evidence and validate its build, presentation, exit, and renderer-report state
• apply the selected runtime environment when launching viewport children
• record configuration-specific provenance for multi-config external builds
• add ost formation env and ost formation doctor
• update the v0.20.0 roadmap, guides, design notes, and generated CLI reference

Why

This closes the remaining v0.20.0 renderer and Formation work. Managed renderer evidence could previously be copied or become stale without completion records detecting it, viewport failures did not expose one durable JSON contract, and viewport child processes were launched without the selected runtime activation environment. Multi-config imports also lacked an explicit selected configuration, while Formation users still needed supported environment export and diagnostics.

Impact

Renderer producers and validators now share digest-bound evidence, viewport automation gets a durable machine-readable launch record, and Windows renderer launches inherit the runtime environment they were resolved against. External Visual Studio, Ninja Multi-Config, and Xcode imports must select an available configuration. Formation users can inspect composed environments and diagnose lock, conflict, and command-reachability problems directly from the CLI.

Validation

• cargo test --workspace
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo fmt --all -- --check
• python scripts/check_docs_consistency.py .
• python scripts/check_doc_links.py .
• git diff --check
• hdMerlin USD viewport dogfood with Vulkan, hidden presentation, and one frame
• ost validate --intent viewport-usd --profile usd --json --redact-paths