Add epd7in3e display driver component

Ports Waveshare's official EPD_7in3e.c register/refresh sequence (the
panel has no public datasheet, so their reference driver is the source of
truth) to an ESP-IDF component using spi_master + gpio instead of the
bcm2835/RPi hardware abstraction the reference targets.

Unlike the reference driver, which toggles CS around every single byte,
this holds CS low for each logical command/data phase and DMAs pixel data
in 4KB chunks -- sending ~192,000 individual one-byte SPI transactions
would make a full refresh impractically slow.

Exposes a streaming API (epd_display_stream, pulling chunks from a
caller-supplied read_fn) so the eventual HTTP fetch path can feed the panel
without holding a full ~192KB frame in RAM, plus an in-memory convenience
wrapper (epd_display_buffer) for cases like the upcoming QR code screen
where buffering the whole frame is fine.

Wired into wifi_provisioning_start() as an init + white-clear for now, to
prove the driver builds/links/runs at the right point in the boot sequence
ahead of the actual QR code content.

firmware/main/wifi_provisioning.c

@@ -19,6 +19,7 @@
 #include "freertos/FreeRTOS.h"
 #include "freertos/task.h"
 
+#include "epd7in3e.h"
 #include "wifi_provisioning.h"
 
 #define NVS_NAMESPACE "frame_cfg"
@@ -370,11 +371,19 @@ void wifi_provisioning_start(void)
     char ap_password[FRAME_AP_PASSWORD_LEN + 1];
     ap_identity_get(ap_ssid, sizeof(ap_ssid), ap_password, sizeof(ap_password));
 
-    /* TODO(step 5): draw the WiFi-join QR code + plaintext password on the
-     * e-ink panel here, before the AP goes up, so the join instructions are
-     * always visible by the time the network is joinable. */
     ESP_LOGI(TAG, "Provisioning AP: SSID='%s' password='%s'", ap_ssid, ap_password);
 
+    /* Bring up the display before the AP so join instructions are on-screen
+     * by the time the network is joinable.
+     * TODO(step 5): replace this clear with the WiFi-join QR code +
+     * plaintext password. */
+    esp_err_t epd_err = epd_init();
+    if (epd_err == ESP_OK) {
+        epd_clear(EPD_COLOR_WHITE);
+    } else {
+        ESP_LOGW(TAG, "EPD init failed (%s), continuing without display", esp_err_to_name(epd_err));
+    }
+
     esp_netif_create_default_wifi_ap();
     wifi_init_softap(ap_ssid, ap_password);