discover: back off the sweep instead of probing every 4 s forever

Reported from a regional packet monitor: a burst of unattributable Control
packets whenever someone had WadaMesh Discover open. Correct on both counts.
They are ours, and they cannot be attributed: a NODE_DISCOVER_REQ carries a type
filter, a random tag and a since-timestamp, and no sender identity at all.

The page swept every 4 seconds for as long as it stayed open, and scanning
defaults to ON when it opens. A probe is not one packet: it is our zero-hop
broadcast PLUS a reply from every node in earshot. So an open Discover page put
a burst on the whole neighbourhood every four seconds, indefinitely.

Worth noting the inconsistency that made this obvious: wada.mesh.discover()
enforces a 15 second floor on third-party apps, with a comment explaining that a
probe spends everyone else's airtime. The built-in was doing it nearly 4x
faster.

Now 4s, 4s, 8s, 15s, then 30s. The first seconds are when someone is actually
looking, so those stay fast and the steady state goes quiet. Opening the page or
pressing Scan resets to the fast cadence, because both mean somebody is looking
again. The existing TX-budget gate is unchanged.

Reported by kevin77.

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

src/ui-touch/UITask.cpp

@@ -22594,14 +22594,35 @@ static void discoverBuildFeed() {
     lv_label_set_text_fmt(s_discover_status, TR("%s \xC2\xB7 %d nearby (%d rpt, %d comp)"), s_discover_scanning ? "Scanning\xE2\x80\xA6" : "Paused", (int)m, rpt, comp);
 }
 
+// Sweep interval, backing off. A probe is not one packet: it is our zero-hop
+// broadcast PLUS a reply from every node that hears it, so a fixed 4 s sweep
+// with the page left open put a burst on the whole neighbourhood every four
+// seconds, for as long as it stayed open. Reported from a regional packet
+// monitor as "a ton of Control packets" that could not be attributed to any
+// node (a discovery REQ carries no sender identity, by design).
+//
+// The answer is not simply "slower": the first seconds are when the user is
+// actually looking, so those stay fast and the steady state goes quiet. Same
+// reasoning as the 15 s floor the Lua SDK enforces on wada.mesh.discover(),
+// which this used to undercut by nearly 4x while being the built-in.
+static const uint32_t kDiscSweepMs[]  = { 4000, 4000, 8000, 15000, 30000 };
+static uint8_t        s_disc_sweep_n  = 0;
+
+static uint32_t discoverSweepInterval() {
+  const uint8_t n = sizeof(kDiscSweepMs) / sizeof(kDiscSweepMs[0]);
+  return kDiscSweepMs[s_disc_sweep_n < n ? s_disc_sweep_n : (n - 1)];
+}
+
 static void discoverTimerCb(lv_timer_t* t) {
   (void)t;
   if (!s_discover_root) return;
   const uint32_t now = millis();
-  if (s_discover_scanning && (s_discover_last_scan_ms == 0 || (now - s_discover_last_scan_ms) >= 4000)) {
+  if (s_discover_scanning &&
+      (s_discover_last_scan_ms == 0 || (now - s_discover_last_scan_ms) >= discoverSweepInterval())) {
     if (the_mesh.getRemainingTxBudget() >= 300) {   // airtime gate: keep a duty-cycle cushion
       the_mesh.uiStartDiscoverScan(0);              // 0 = all node types
       s_discover_last_scan_ms = now;
+      if (s_disc_sweep_n < 250) s_disc_sweep_n++;
     }
   }
   discoverBuildFeed();
@@ -22611,7 +22632,10 @@ static void discoverTimerCb(lv_timer_t* t) {
 static void discoverScanToggleCb(lv_event_t* e) {
   if (lv_event_get_code(e) != LV_EVENT_CLICKED) return;
   s_discover_scanning = !s_discover_scanning;
-  if (s_discover_scanning) s_discover_last_scan_ms = 0;   // resume -> sweep immediately
+  if (s_discover_scanning) {
+    s_discover_last_scan_ms = 0;   // resume -> sweep immediately...
+    s_disc_sweep_n = 0;            // ...and at the fast cadence again
+  }
   if (s_discover_btn_lbl) lv_label_set_text(s_discover_btn_lbl, s_discover_scanning ? "Stop" : "Scan");
   discoverBuildFeed();
 }
@@ -22653,6 +22677,7 @@ static void closeDiscoverPage() {
   s_discover_feed = s_discover_status = s_discover_btn_lbl = s_disc_footer = nullptr;
   s_disc_row_n = 0;
   s_discover_scanning = true;
+  s_disc_sweep_n = 0;   // a fresh look starts fast, then backs off
   if (s_apppage_close == closeDiscoverPage) {
     s_apppage_title = nullptr; s_apppage_close = nullptr;
     statusBarSetTall(false); updateGlobalStatusBar();
@@ -22733,6 +22758,7 @@ static void openDiscoverPage() {
 
   the_mesh.discoverClear();
   s_discover_scanning = true;
+  s_disc_sweep_n = 0;   // a fresh look starts fast, then backs off
   s_discover_last_scan_ms = 0;                // fire the first sweep on the first tick
   discoverBuildFeed();
   s_discover_timer = lv_timer_create(discoverTimerCb, 1000, nullptr);