Advance v0.20.0 dogfooding contracts

[snkmcb]

Summary

• make aggregate plugin products reproducible and add first-class product verify / product install workflows
• add renderer capability preflight plus actionable timeout and target-lease recovery
• capture the v0.20.0 dogfooding scope while explicitly deferring DCC integrations to v0.21.0

Why

The v0.19.0 downstream dogfooding reports exposed two release-blocking contract gaps:

• aggregate product digests changed across repeated packaging because embedded member manifests used wall-clock creation timestamps
• renderer workflows could discover missing USD capabilities only after entering the build or launch path

The reports also requested more actionable timeout recovery and portable activation outside an OpenStrata-owned process.

Impact

• unchanged aggregate products now produce stable digests, with SOURCE_DATE_EPOCH support
• product archives can be independently verified and atomically installed with aggregate activation entrypoints and receipts
• USD viewport intent can be checked before building, with typed requested/applied/skipped/unrequested capability evidence
• handled build failures release target leases, while timeout diagnostics name the phase, PID, command, working directory, log, cleanup result, and last output
• package dependency closure and non-OST activation are covered by regression tests and real downstream dogfooding

DCC-specific integrations remain out of scope for v0.20.0 and are tracked for v0.21.0.

Validation

• cargo test --workspace --locked
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo fmt --all -- --check
• cargo run -q -p ost-cli --locked -- internal docs generate
• python scripts/check_docs_consistency.py .
• python scripts/check_doc_links.py .
• git diff --check
• usd-vrm-plugins bare --from-package L0-L4 dogfood
• usd-vrm-plugins activation from plain OpenUSD CLI and stock Python
• hydra-merlin renderer preflight with both compatible and incompatible profiles