test: add sim-yellow and sim-red envs for visual border verification

Two pio envs that build a tiny sketch reading /img.bin from LittleFS and
calling epd_draw_image_with_border with the chosen color. Lets us verify
the actual on-device pixel composition of the sync-fail (yellow) and
no-WiFi (red) borders without standing up a server failure or pulling
the WiFi cable.

Each sim sets NVS err_border=1 before halting, so flashing back to the
normal env afterwards exercises the 304 → clean repaint recovery path
end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/sim_border.cpp

@@ -0,0 +1,69 @@
+// Visual hardware test for sync-fail / no-WiFi border rendering.
+//
+// Built only by env:sim-yellow and env:sim-red in platformio.ini. Reads the
+// last cached image from LittleFS, draws it through epd_draw_image_with_border
+// with the configured color, and sets NVS_KEY_ERR_BORDER=1 so that flashing
+// the normal firmware afterwards exercises the recovery path (next 304
+// repaints the image clean).
+//
+// Usage:
+//   pio run -e sim-yellow --target upload
+//   pio run -e sim-red    --target upload
+//   pio run -e esp32dev   --target upload   # back to normal — verifies recovery
+
+#ifdef SIM_BORDER
+
+#include <Arduino.h>
+#include <SPI.h>
+#include <LittleFS.h>
+#include <Preferences.h>
+#include "config.h"
+#include "epd.h"
+
+#ifndef SIM_BORDER_COLOR
+#error "sim_border.cpp built without SIM_BORDER_COLOR — set it in platformio.ini"
+#endif
+
+void setup() {
+    Serial.begin(115200);
+    Serial.println("[sim_border] boot");
+
+    pinMode(PIN_CS,        OUTPUT);
+    pinMode(PIN_DC,        OUTPUT);
+    pinMode(PIN_RST,       OUTPUT);
+    pinMode(PIN_BUSY,      INPUT);
+    pinMode(PIN_BTN_RESET, INPUT_PULLUP);
+
+    SPI.begin(PIN_SCK, -1, PIN_MOSI, PIN_CS);
+    SPI.beginTransaction(SPISettings(4000000, MSBFIRST, SPI_MODE0));
+    LittleFS.begin(true);
+
+    epd_init();
+
+    File f = LittleFS.open(IMAGE_PATH, "r");
+    if (f) {
+        Serial.printf("[sim_border] /img.bin %u bytes — drawing with border color 0x%X\n",
+                      (unsigned)f.size(), (unsigned)SIM_BORDER_COLOR);
+        epd_draw_image_with_border(f, SIM_BORDER_COLOR, BORDER_THICKNESS_PX);
+        f.close();
+    } else {
+        Serial.println("[sim_border] no /img.bin — falling back to full fill");
+        epd_fill(SIM_BORDER_COLOR);
+    }
+
+    epd_sleep();
+
+    // Mark err_border so the next normal-firmware boot will trigger the
+    // 304-based clean repaint and we can confirm recovery end-to-end.
+    Preferences prefs;
+    prefs.begin(NVS_NAMESPACE, false);
+    prefs.putInt(NVS_KEY_ERR_BORDER, 1);
+    prefs.end();
+
+    Serial.println("[sim_border] done — halting (deep sleep, button-reset to wake)");
+    esp_deep_sleep_start();
+}
+
+void loop() {}
+
+#endif // SIM_BORDER