Watch transitive proto imports in build

[fallintoplace]

Summary

• Make Protoc mode emit Cargo rerun watches from the decoded descriptor set, so resolved transitive imports trigger codegen again.
• Keep explicitly listed input files watched, and skip descriptor names that do not resolve safely under configured include roots.
• Add unit coverage and a changelog fragment for the build rerun behavior.

Validation

• cargo test --workspace --all-features
• cargo clippy --workspace --all-targets --all-features -- -D warnings

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}