fix: explain missing CC2 source bundle

[albertoelopez]

Summary

• improves CC2 board source-bundle discovery diagnostics
• lists searched .omx candidate paths when required artifacts are missing
• supports CC2_SOURCE_OMX for explicit source bundle selection
• keeps current workspace clippy gate green against current main

Test Plan

• python3 -m unittest discover -s tests -v
• cd rust && ../scripts/fmt.sh --check
• cd rust && cargo clippy --workspace --all-targets -- -D warnings
• cd rust && cargo test --workspace

Notes

The board generator still requires the frozen CC2 source bundle. This change makes the failure actionable instead of silently guessing or fabricating source artifacts.

[albertoelopez]

Maintainer merge request: this PR is currently mergeable and has no reported failing checks. I attempted to merge it, but GitHub returned a permission error for my account (albertoelopez does not have the correct permissions to execute MergePullRequest). Could someone with write access please review/merge when convenient? Test plan is included in the PR body.

[1716775457damn]

Good improvement on diagnostics here. The explicit CC2_SOURCE_OMX env var for source bundle selection is a sensible escape hatch — much better than silently guessing.

One thought: would it be worth logging the searched paths at the warn level by default (before failure), so users can see potential mismatches earlier in their workflow rather than only on error?

Overall looks clean, and keeping clippy green is appreciated. Nice work.