diff --git a/requirements.txt b/requirements.txt
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+# Build dependencies for the handbook docs site.
+# Host build: python -m venv .venv && . .venv/bin/activate
+#             pip install -r requirements.txt && mkdocs build
+# Output: static site/ served by the reverse proxy at docs.rethinkstudios.io.
+
+mkdocs-material==9.7.6            # theme — pulls mkdocs, pymdown-extensions, pygments
+mkdocs==1.6.1                     # pinned explicitly for a reproducible build
+mkdocs-material-extensions==1.3.1
+pymdown-extensions==11.0          # admonitions, tabbed, superfences, annotations
+Pygments==2.20.0                  # syntax highlighting in code blocks