test: add standalone FastLED rainbow strip sketch

Tools/Developer/FastLedRainbow/FastLedRainbow.ino

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+// SPDX-FileCopyrightText: 2026 PCController contributors
+// SPDX-License-Identifier: MIT
+#include <FastLED.h>
+
+constexpr uint8_t DataPin = PIN_PD6; // Arduino D6, PCController strip data
+constexpr uint8_t PixelCount = 100;
+CRGB pixels[PixelCount];
+uint8_t hue = 0;
+uint32_t frames = 0;
+uint32_t lastReport = 0;
+
+void setup() {
+  // Disable the PCController's unrelated active-low shift-register outputs.
+  digitalWrite(PIN_PB0, HIGH);
+  pinMode(PIN_PB0, OUTPUT);
+  Serial.begin(115200);
+  FastLED.addLeds<WS2811, DataPin, BRG>(pixels, PixelCount);
+  FastLED.setBrightness(64);
+  FastLED.clear(true);
+  Serial.println(F("FastLED rainbow: D6/PD6, WS2811, BRG, 100 pixels, brightness 64"));
+}
+
+void loop() {
+  fill_rainbow(pixels, PixelCount, hue++, 7);
+  FastLED.show();
+  ++frames;
+  if (millis() - lastReport >= 2000) {
+    lastReport = millis();
+    Serial.print(F("rainbow frames="));
+    Serial.println(frames);
+  }
+  delay(20);
+}

Tools/Developer/FastLedRainbow/README.md

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+# FastLED rainbow hardware test
+
+Standalone LED test for the MiniCore ATmega328P PCController board:
+D6/PD6, 100 WS2811 pixels, BRG color order, brightness 64/255, animated rainbow.
+The active-low 74HC595 output enable on D8/PB0 is held disabled. Other controller
+features are absent while this sketch is installed. This does not modify EEPROM.
+Serial at 115200 baud reports startup configuration and frame count every two seconds.
+Frame reports establish firmware execution, not physical LED output.
+
+Requires MiniCore and FastLED (verified with MiniCore 3.1.3, FastLED 3.10.5).
+Compile and upload using Arduino CLI, replacing COM3 with the actual board port:
+
+```powershell
+arduino-cli compile --fqbn MiniCore:avr:328:clock=16MHz_external,variant=modelP,bootloader=uart0 --build-path work/rainbow-build Tools/Developer/FastLedRainbow
+arduino-cli upload --fqbn MiniCore:avr:328:clock=16MHz_external,variant=modelP,bootloader=uart0 --port COM3 --input-dir work/rainbow-build Tools/Developer/FastLedRainbow
+```
+
+This replaces the controller application. Preserve a board-local flash/EEPROM
+backup before uploading if the installed firmware must be recoverable. Keep
+those device backups private. Restore normal PCController firmware when done.
+A WS2812B strip instead needs WS2812B and GRB in the addLeds template.