docs(runtime): add sourceos-shell PDF stack note

Author: mdheller

docs/sourceos-shell-pdf-stack.md

@@ -0,0 +1,26 @@
+# sourceos-shell PDF stack note
+
+This note tracks the Linux realization scaffold for the PDF lane during the `sourceos-shell` rollout.
+
+## Current Linux-facing surfaces
+
+The Linux realization currently carries service scaffolds for:
+
+- `sourceos-pdf-secure`
+- `sourceos-docd`
+
+These are represented both as systemd unit files under `linux/systemd/` and as service declarations in `modules/nixos/sourceos-shell/default.nix`.
+
+## Intent
+
+This is the Linux realization slice of the broader PDF lane tracked in `#93`.
+
+The future `SourceOS-Linux/sourceos-shell` runtime repo should own the actual PDF derivation, signing, validation, and viewing behavior. `source-os` should only keep the host/service realization and validation hooks.
+
+## Validation scaffold
+
+The current Linux realization adds a dedicated contract-style check at:
+
+- `tests/sourceos-shell-pdf-stack-contract.nix`
+
+This check verifies that the PDF-related runtime scaffolds and module hooks remain present and aligned.