improved timing acq

README.md

@@ -699,6 +699,17 @@ now — they are listed so they are not mistaken for anything else:
 - **The time reference is the first *enabled* channel's signal.** A CONFIG whose
   time signal belongs to a signal that is disabled contributes no stream time, and
   the app falls back to arrival-based timing (with a warning on stderr).
+- **The choice of time source is sticky, and a change of source empties the rings.**
+  It is decided once (and re-decided on a CONFIG) and then kept: choosing per frame
+  let a stream whose declared time signal fails one check and passes the next
+  alternate between two timelines, so one ring held stamps from both and the x axis,
+  the crosshair and an export read nonsense — a plateau of one timestamp, then a jump
+  to another epoch. On the switch the recorded history is dropped rather than left
+  half on each clock. The stamps themselves are spaced by the stream's *measured*
+  arrival cadence whenever the declared rate disagrees with it, because a CONFIG's
+  rate is a claim the stream may not keep (one that declared 1 MS/s and delivered a
+  hundred samples every fifth of a second made every frame span microseconds while
+  the frames were 100 ms apart).
 
 ## Limitations