Render diagrams with local provider icon packs

[konojunya]

Summary

• add repeatable --provider-pack <DIRECTORY> loading to stack render
• add --notice <NOTICE> for artifact-specific provider provenance and used-icon records
• reject symlinked, oversized, unsafe-path, malformed, or hash-mismatched local pack inputs before rendering
• pin Engine 0.4, the final namespaced identifier specification, and the final compiler revision
• document the local-only render flow with a fictional process fixture

No vendor artwork is committed or distributed, and the command performs no download or upload.

Validation

• stable and Rust 1.85 tests and Clippy
• formatter conformance against specification 80df39151df67eedb4cfcb2de9f964129cd87466
• rustdoc and release build
• standalone SVG validation
• 92.47% regions, 97.97% functions, 93.08% lines
• local end-to-end import and one render using audited AWS, Google Cloud, and Azure archives; the resulting SVG resolved all three namespaced icons and the sidecar recorded the exact three pack revisions and terms links