v5.9: the latency control governs the streams it's supposed to, and arrives in seconds

Confirmed working on Ed's two machines before shipping (his log: slider 500 with 17ms buffered
at 07:10:31 -> 214ms at +3s -> 445 at +9s -> settled ~490 by +15s; auto-tune then walked it down
in 5ms steps with the buffer tracking; a drop to 20ms took effect immediately). Both directions,
auto-tune on and off.

Third fix in this batch, found while answering "is ASIO testing essential before we ship": the
ASIO slider only exists in BothIndependent, but MainForm pushes its persisted value at startup in
EVERY mode (and its auto-tune can tick). Now that all routes resolve to the shared value in
single-slider mode, that write would silently overwrite the visible slider with a hidden control's
number. SetMaxLatencyMs now ignores AsioLane writes when !independentLanes — in single-slider mode
the ASIO box governs nothing. Gate covers it, plus the ASIO half of the wiring (a stream on the
ASIO lane reads the ASIO slider; moving one lane doesn't disturb the other) — provable without an
ASIO device, since the change is about WHICH value a lane reads, not the driver path (untouched).

Docs: manual gains an honest paragraph on changing latency mid-listen (immediate down, a few
seconds up, the slight stretch is the change happening, no gap or click); About + RELEASE_NOTES
for 5.9; MANUAL regenerated. Version 5.9. Gate 74/74 + relay 7.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

MANUAL.md

@@ -736,6 +736,8 @@ Separately from the controls above — which manage the cushion against _network
 
 The **Audio latency** control tells the receiving side how much sound to keep in reserve as a cushion against uneven network timing. A bigger cushion means more delay but fewer clicks. A smaller cushion means less delay but more clicks when the network wobbles.
 
+**You can change it while you're listening.** Move the control and the delay follows within a few seconds — there's no gap or click while it changes. Lowering it takes effect straight away. Raising it can't happen instantly, because the extra cushion has to be built up out of the sound still arriving, so RemSound plays very slightly slow for a moment while it banks the difference: a big jump takes a few seconds to arrive and you can hear it stretch out as it goes. That's the change happening, not a fault.
+
 Setting| Best for| Trade-off
 ---|---|---
 5–10 ms| Local network, same computer.| Crackles on any internet connection with even modest jitter.