Add HAI-DEF NOTICE file (adapter Model Derivative notice propagation)

NOTICE

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+CXR Draft Auditor
+Copyright (c) 2026 CXR Draft Auditor contributors.
+
+This product includes a Model Derivative of MedGemma, a Health AI Developer
+Foundations (HAI-DEF) model published by Google.
+
+HAI-DEF is provided under and subject to the Health AI Developer Foundations
+Terms of Use found at:
+https://developers.google.com/health-ai-developer-foundations/terms
+
+The Prohibited Use Policy incorporated into those Terms is found at:
+https://developers.google.com/health-ai-developer-foundations/prohibited-use-policy
+
+Base model: google/medgemma-1.5-4b-it (HAI-DEF license).
+
+This repository distributes the unmerged QLoRA LoRA adapter weights of that
+Model Derivative, trained from the base model named above and pushed as a
+pre-merge safety copy. Any file modified relative to the upstream MedGemma
+distribution carries a prominent "modified" notice. Recipients of this Model
+Derivative are subject to the Section 3.2 use restrictions of the HAI-DEF Terms
+of Use, including the Clinical Use restriction; the full Agreement is provided
+with any distribution and is referenced above.
+
+This is research and educational software only. It is NOT a medical device, NOT
+diagnosis, and NOT for clinical use. See the model card (README.md).
+
+Google does not endorse this Model Derivative, this software, or its authors.
+"MedGemma", "Gemma", and "Google" are trademarks of Google LLC and are used here
+only for accurate attribution of the base model; no trademark license or
+endorsement is granted or implied.