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>

DEMO.md

@@ -1,6 +1,6 @@
 # The enrollment demo: what it is, what it does, what it checks
 
-A throwaway prototype that proves the board enrollment design works across three languages and a real broker. It covers **Stage 0 and Stage 1 only** (a raw board becomes an FN board and uses its FN topics). Activation (FN to SN) is described in `docs/investigations/mqtt-board-identity/sn-as-username-process.md` in the backend repo but is **not built here**.
+A throwaway prototype that proves the board enrollment design works across three languages and a real broker. It covers **Stage 0 and Stage 1 only** (a raw board becomes an FN board and uses its FN topics). Activation (FN to SN) is described in [docs/sn-as-username-process.md](docs/sn-as-username-process.md) but is **not built here**.
 
 Everything is in `crypto-interop-spike/`.