Bus input: serve envelopes from yukart/vehicle/+ and yukart/host/+

Newest per signal, stale after 5 s or when only the retained copy is known.
Bus or mock, never both; hello says which (source bus|mock|none).
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz

docs/impl/plans/INITIAL.md

@@ -1,7 +1,7 @@
 ---
 type: plan
 status: active
-updated: 2026-10-09
+updated: 2026-10-12
 ---
 # Initial plan
 
@@ -23,6 +23,9 @@ directly.
 0. **Boundary v0** — in place: WebSocket on `:7700`, hello, heartbeats both ways,
    `mark` forwarded to the recorder's control socket, mock state behind `--mock`.
    Protocol in contracts [`docs/BOUNDARY.md`](gitea:yukart/contracts).
+   **Bus input** — in place (2026-10-12): subscribes to `yukart/vehicle/+` and
+   `yukart/host/+`, keeps the newest envelope per signal, marks stale after 5 s
+   or when known only from a retained copy. Bus or mock, never both.
 1. Static file serving for the basemap — smallest useful thing, unblocks the map.
 2. Nav HTTP proxy to [svc-nav](gitea:yukart/svc-nav).
 3. Aggregation and rate limiting, once there is real CAN traffic to aggregate.