Record link shapes and boundary semantics in initial plan

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013kj58Jugkic783b8ZrtTVZ

docs/impl/plans/INITIAL.md

@@ -30,3 +30,22 @@ worth having before any navigation UI exists.
 Live guidance needs a GNSS fix, which needs gpsd, which needs the SIM7600 brought
 up in [orin-nano-env-setup](gitea:yukart/orin-nano-env-setup). Everything else here
 can be built and validated against a replayed trace.
+
+## Links
+
+| Link | Shape | Initiator |
+|---|---|---|
+| `yukart/gnss/fix` | subscribe | — |
+| `yukart/nav/{guidance,route}` | publish, retained | — |
+| Route requests | request/response, HTTP | `svc-gw` |
+| Cloud routing API, if used | request/response | this service, outbound only |
+
+`svc-nav` never talks to a display. Guidance reaches the app as bus state via
+`svc-gw`, and route requests arrive proxied. Nav does not know displays exist.
+
+## GNSS is scaffolding for now
+
+The PCBs are not assembled, so there is no real fix yet. Positions come from a
+replayed trace publishing `yukart/gnss/fix` with the **real schema** — see the
+fixtures rule in [ARCHITECTURE.md](gitea:yukart/project-yukart). Everything
+downstream is then identical when the hardware lands.