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]

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[iainmcgin]

[claude code] Thanks @fallintoplace — clean, correct, and well-tested. Verified end-to-end: the descriptor set carries the transitive closure (--include_imports is unconditional), the resolver's include-root iteration deliberately mirrors protoc's -I order, explicitly-listed files stay watched unconditionally, and the traversal guard is sound. Pushed a small fixup (d59ec3a): doc notes for the deliberate WKT non-watch and the resolver's guards, plus changelog fragment polish. Will queue for merge once CI runs.

[iainmcgin]

[claude code] Merged as 88e854f — thanks @fallintoplace!