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

@@ -24,3 +24,24 @@ needs no routing engine at all. Routing is the expensive last fraction.
   font rendering must be configured, or labels will be slow or absent.
 - **Sunlight.** A typical tablet is unreadable through a windscreen in daylight.
   This is a hardware problem, but the UI should not make it worse.
+
+## The boundary, from this side
+
+Two channels with different guarantees, not one socket:
+
+| | Down (state) | Up (commands) |
+|---|---|---|
+| Transport | push stream | request/response |
+| Loss | tolerated; next sample supersedes | **not** tolerated; retry until acknowledged |
+| On reconnect | expect current state immediately | resend anything pending |
+
+This client always initiates and owns reconnection — the Orin never dials out.
+Assume the link drops constantly: cable knocked, screen off, WiFi lost.
+
+## Session marking is worth building early
+
+A button that stamps an event (`indicator on`, `cold start`, `hard braking`) into
+the recording, on the same clock as the CAN frames, turns the Phase 1 manual event
+log from a notebook-and-wristwatch exercise into structured data. It is what makes
+a raw capture decodable later, and it is a Phase 1 capability that happens to live
+in this repo.