e1608: a configurable generator update rate, and the achieved rate in the Analog out panel

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/claude/architecture.md

@@ -51,7 +51,7 @@ What feeds `Acquisition` is a `DataSource`, one of the registered *kinds*.
   never reaches the UI files.
 - The E-1608 source runs **everything inside `poll()`** on the producer thread: posted commands, the
   connect state machine (Down, Scanning, Halted), the scan read, the 500 ms status check (its not-running restart is skipped when a hardware trigger is configured), the digital
-  input/counter poll and the 20 ms waveform tick. There is no second thread. A connect attempt (discovery,
+  input/counter poll and the waveform tick (every `1 / gen_rate` s, 20 ms by default; one synchronous DAC write per running channel, so a slow board lowers the rate it achieves, which the Analog out panel reports). There is no second thread. A connect attempt (discovery,
   handshake, calibration read, scan start) can block `poll()` for as long as the command timeout allows each
   step; that delays only the command queue. Every exception is caught inside `poll()`: an escape would end
   the producer thread, and `drain_commands()` runs inside the same `try` for that reason.