[Stage 3] Design CI artifact directory fixture layout

[Will-thom]

Goal: design fixture layouts for scanning a CI artifact directory separate from the source root.

Related plan: #82.

Depends on: #36 and #79.

Acceptance criteria:
- Define fixture layouts for source root plus artifact directory.
- Cover module mapping when artifacts are outside module target directories.
- Document failure modes when mapping is ambiguous.
- Keep tests local and fixture-driven.

[github-actions]

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