Drive splash animations by session usage rate

Group splash animations into four moods (idle/normal/active/heavy) and pick
which group plays based on a 5-min sliding rate of session_pct. Thresholds
sit at 0.10/0.20/0.33 %/min so Heavy fires when usage matches or beats the
5-hour session reset pace. Splash auto-rotates within the current group
every 20s; group recomputes on every BLE poll and the splash re-picks
immediately if it changes mid-display. 4-min minimum window prevents
single-sample bumps after boot from falsely landing in Heavy.

firmware/src/main.cpp

@@ -8,6 +8,7 @@
 #include "power.h"
 #include "imu.h"
 #include "splash.h"
+#include "usage_rate.h"
 
 // Physical buttons (global, screen-independent):
 //   BTN_BACK   (GPIO 0)  — left,  send Space (Claude Code voice mode push-to-talk)
@@ -402,6 +403,14 @@ void loop() {
     // Process incoming BLE data
     if (ble_has_data()) {
         if (parse_json(ble_get_data(), &usage)) {
+            int g_before = usage_rate_group();
+            usage_rate_sample(usage.session_pct);
+            int g_after = usage_rate_group();
+            if (g_after != g_before) {
+                Serial.printf("usage rate: group %d -> %d (s=%.2f%%)\n",
+                    g_before, g_after, usage.session_pct);
+                if (splash_is_active()) splash_pick_for_current_rate();
+            }
             ui_update(&usage);
             ble_send_ack();
         } else {