feat(firmware): gate LED gamma viz behind CONFIG_LED_GAMMA_VIZ (ADR-183 follow-up) (#1129)

The 40 Hz gamma flicker is now Kconfig-gated (default y, unchanged
behaviour). Set CONFIG_LED_GAMMA_VIZ=n for a dark, lower-power boot (the
LED is simply cleared) — important for photosensitive deployments, no
source edit needed. The colormap saturation point is now operator-tunable
via CONFIG_LED_MOTION_FULLSCALE_MILLI (default 250 = 0.25).

Build + flash confirmed on ESP32-S3 N16R8 (COM8): default y still arms the
gamma timer, CSI flows. ADR-183 updated (gate moved from follow-up to done).

docs/adr/ADR-183-onboard-led-gamma-stimulus-csi-colormap.md

@@ -74,11 +74,13 @@ therefore provably `ruv-neural-viz`'s, and the motion is provably real.
 - Confirms #962's GPIO-48 fix visually (the LED lights on N16R8).
 
 **Negative / risks**
-- Changes *default* firmware behaviour: the onboard LED now animates instead of staying
+- Changes the *default* firmware behaviour: the onboard LED animates instead of staying
-  off (minor power + a visible flicker some may not want). Gate behind a Kconfig
+  off. Now **gated by `CONFIG_LED_GAMMA_VIZ`** (default `y`); set it `n` for a dark,
-  (`CONFIG_LED_GAMMA_VIZ`) if a dark default is preferred — follow-up.
+  lower-power boot (the LED is just cleared) — no source change needed.
-- A 40 Hz flicker can be an issue for photosensitive users; document on the enclosure.
+- A 40 Hz flicker can be an issue for photosensitive users; document on the enclosure
-- `LED_MOTION_FULLSCALE` (0.25) is hand-tuned, not calibrated per-environment.
+  and disable `CONFIG_LED_GAMMA_VIZ` in those deployments.
+- The saturation point is now `CONFIG_LED_MOTION_FULLSCALE_MILLI` (default 250 = 0.25),
+  operator-tunable; still not auto-calibrated per-environment.
 - The colour uses a baked LUT, not the live Rust `ColorMap` (FFI path deferred — needs
   the ESP Rust/xtensa toolchain, not yet in CI).
 

firmware/esp32-csi-node/main/Kconfig.projbuild

@@ -468,3 +468,29 @@ menu "Mock CSI (QEMU Testing)"
         depends on CSI_MOCK_ENABLED
         default n
 endmenu
+
+menu "Onboard LED (ADR-183)"
+
+    config LED_GAMMA_VIZ
+        bool "Onboard WS2812: 40 Hz gamma flicker + CSI-motion colour"
+        default y
+        help
+            Drive the onboard WS2812 as a GENUS-style 40 Hz gamma square wave
+            (12.5 ms on / 12.5 ms off, 50% duty). The ON-phase colour is live
+            CSI motion (edge motion_energy) mapped through the ruv-neural-viz
+            viridis colormap (still=purple, moving=yellow).
+
+            Disable to leave the LED off at boot — lower power, no flicker.
+            NOTE: a 40 Hz flicker can affect photosensitive users; disable or
+            shield the LED in those environments. Not a medical device.
+
+    config LED_MOTION_FULLSCALE_MILLI
+        int "Motion value (x1000) that saturates the colormap to yellow"
+        depends on LED_GAMMA_VIZ
+        default 250
+        range 1 100000
+        help
+            edge motion_energy that maps to the top (yellow) of the viridis
+            colormap, in milli-units (250 = 0.25). Lower = more sensitive
+            (reaches yellow with less motion).
+endmenu

firmware/esp32-csi-node/main/main.c

@@ -144,6 +144,7 @@ static void wifi_init_sta(void)
     }
 }
 
+#if CONFIG_LED_GAMMA_VIZ
 /* Viridis colormap (60 steps), generated from ruv-neural-viz::ColorMap::viridis()
  * — the rUv-Neural brain-topology colormap, now no_std (ruvnet/ruv-neural#3 /
  * RuView#1126). Used as the ON-phase colour of the 40 Hz gamma flicker below:
@@ -164,8 +165,8 @@ static const uint8_t VIRIDIS_LUT[60][3] = {
 };
 static led_strip_handle_t s_viz_led;
 
-/* motion_energy that saturates the colormap to yellow (tunable). */
+/* motion_energy that saturates the colormap to yellow (CONFIG, milli-units). */
-#define LED_MOTION_FULLSCALE 0.25f
+#define LED_MOTION_FULLSCALE ((float)CONFIG_LED_MOTION_FULLSCALE_MILLI / 1000.0f)
 
 /* GENUS-style 40 Hz gamma flicker: full on/off square wave, 50% duty (toggled
  * every 12.5 ms → 40 Hz). The ON colour is live CSI motion (edge motion_energy)
@@ -189,6 +190,7 @@ static void led_gamma_40hz_cb(void *arg)
     }
     led_strip_refresh(s_viz_led);
 }
+#endif /* CONFIG_LED_GAMMA_VIZ */
 
 void app_main(void)
 {
@@ -219,10 +221,11 @@ void app_main(void)
     ESP_LOGI(TAG, "%s CSI Node (ADR-018 / ADR-110) — v%s — Node ID: %d",
              target_name, app_desc->version, g_nvs_config.node_id);
 
-    /* Onboard WS2812: sweep the ruv-neural-viz viridis colormap at 40 Hz.
+    /* Onboard WS2812. C6 wires the LED to GPIO 8; S3 to GPIO 38 (DevKitC-1 v1.0)
-     * C6 dev boards wire the LED to GPIO 8; S3 boards to GPIO 38 (DevKitC-1 v1.0)
      * or GPIO 48 (DevKitC-1 v1.1 / N16R8 — see #962). On S3 we drive 48 (the
-     * common module). On C6, GPIO 38/48 don't exist (only 0-30) — gate by target. */
+     * common module). On C6, GPIO 38/48 don't exist (only 0-30) — gate by target.
+     * Behaviour is set by CONFIG_LED_GAMMA_VIZ (ADR-183): on = 40 Hz gamma flicker
+     * coloured by CSI motion; off = clear the LED at boot. */
 #if defined(CONFIG_IDF_TARGET_ESP32C6)
     const int led_gpio = 8;
 #else
@@ -239,6 +242,7 @@ void app_main(void)
         .resolution_hz = 10 * 1000 * 1000, // 10MHz
         .flags.with_dma = false,
     };
+#if CONFIG_LED_GAMMA_VIZ
     if (led_strip_new_rmt_device(&strip_config, &rmt_config, &s_viz_led) == ESP_OK) {
         const esp_timer_create_args_t viz_args = {
             .callback = &led_gamma_40hz_cb,
@@ -250,6 +254,14 @@ void app_main(void)
             ESP_LOGI(TAG, "Onboard WS2812: 40 Hz gamma flicker (GENUS), colour=CSI motion via ruv-neural-viz, GPIO %d", led_gpio);
         }
     }
+#else
+    /* Viz disabled — clear the onboard LED at boot and release the RMT channel. */
+    led_strip_handle_t led_strip;
+    if (led_strip_new_rmt_device(&strip_config, &rmt_config, &led_strip) == ESP_OK) {
+        led_strip_clear(led_strip);
+        led_strip_del(led_strip);
+    }
+#endif /* CONFIG_LED_GAMMA_VIZ */
 
     /* ADR-110 P4: 802.15.4 mesh time-sync (C6 only).
      * Initialized BEFORE WiFi so it's available even when WiFi STA can't