A technical summary written for a non-specialist, design documents and version-control history naming her as author, internal training material she wrote, and a letter from the engineering lead describing how long a new hire needs to learn the system. Source code itself is not required.
Documents that prove authorship and the learning curve
Start with a plain-language description of the pipeline: what it ingests, what it produces, and which stages embody the company's own methods rather than off-the-shelf tools. Redact algorithms but keep structure. Add extracts from the repository history showing her commits to the core modules over time, design review records, and architecture diagrams with her name in the author field.
Internal onboarding guides she authored show that others learn the system from her. A short letter from the engineering lead should estimate the time a competent external hire would need to reach her level and explain why. Then add the subsidiary's evidence: the dealer contract showing that the subsidiary controls and supervises the work, the subsidiary's own organizational chart placing her under its engineering manager, and a description of deliverables.
Corporate records, payroll evidence for her qualifying year, and both companies' proof of business activity complete the file. Keep everything consistent; an org chart that shows her reporting to the dealer contradicts the rest.