Update the provider pack theme contract

[konojunya]

Summary

• pin the merged Stack Theme 0.3 provider-pack contract
• refresh embedded catalog version and revision expectations
• regenerate deterministic SVG snapshots with the unchanged 12 core icons
• record that vendor asset bytes remain outside Engine and its packages

Validation

• cargo fmt --all --check
• cargo test --workspace --locked
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• conformance render snapshots against the canonical specification
• python3 scripts/validate-svg.py
• git diff --check