docs: simplify the README and record the pad layout

The README carried a lot of rationale that AGENTS.md already holds; what is
left is the tables a consumer wires against. It gains the swept axis and
button indices, and says plainly that the axes check themselves at runtime
and the buttons cannot.

dora-node.yml

@@ -53,6 +53,12 @@ inputs:
       pad node is indistinguishable from an unplugged pad as far as the arm is
       concerned.
 
+      Each pad is measured the first time it is seen sitting still, so a stick
+      that no longer centres has its tilt subtracted rather than deadzoned away.
+      A pad whose resting axes do not fit the layout this node compiles in — a
+      trigger where a stick should be, say — reads the same as no pad at all
+      and is named in the log, rather than being guessed at against a real arm.
+
       Leave it unwired for a browser-only console. The node says so and starts.
   response:
     required: false