Add the design docs to the repo

Copies the SN-as-username process, the FN/SN comparison and the original
R&D notes from the backend repo, links them from the README and stops
documentation-only pushes from redeploying the demo.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -6,6 +6,12 @@ X25519 key agreement so the broker password is never transmitted.
 
 - **[DEMO.md](DEMO.md)**: what the demo is, what it does and the 19 checks each board performs.
 - **[docker/README.md](docker/README.md)**: how to run it and read the web view.
+- **Design docs** (copied from the backend repo, where they live under `docs/investigations/`; links to other
+  backend-repo files appear there as plain text):
+  - [docs/sn-as-username-process.md](docs/sn-as-username-process.md): the full raw board to FN to SN process with SN as the
+    broker username, with the threat model.
+  - [docs/enrollment-and-activation.md](docs/enrollment-and-activation.md): the FN-as-username and SN-as-username models compared.
+  - [docs/mqtt-dynamic-security-rnd.md](docs/mqtt-dynamic-security-rnd.md): the original R&D notes.
 - `go/`, `c/`, `c-libsodium/`, `cpp/`, `run.sh`, `run-libsodium.sh`: fixed-key programs that check the
   crypto libraries against the RFC test vectors and against each other.