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/components/epd7in3e/include/epd7in3e.h

@@ -0,0 +1,47 @@
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+#include "esp_err.h"
+
+/* Waveshare 7.3" E Ink Spectra 6 (E6) panel: 800x480, 4 bits/pixel packed
+ * 2-pixels-per-byte, per the official EPD_7in3e.c reference driver. */
+#define EPD_WIDTH 800
+#define EPD_HEIGHT 480
+#define EPD_BYTES_PER_ROW ((EPD_WIDTH + 1) / 2)
+#define EPD_FRAME_BYTES (EPD_BYTES_PER_ROW * EPD_HEIGHT)
+
+typedef enum {
+    EPD_COLOR_BLACK = 0x0,
+    EPD_COLOR_WHITE = 0x1,
+    EPD_COLOR_YELLOW = 0x2,
+    EPD_COLOR_RED = 0x3,
+    EPD_COLOR_BLUE = 0x5,
+    EPD_COLOR_GREEN = 0x6,
+} epd_color_t;
+
+/** Configures SPI + GPIO and runs the panel's power-on register init sequence. */
+esp_err_t epd_init(void);
+
+/** Fills the whole panel with a single color and refreshes. */
+esp_err_t epd_clear(epd_color_t color);
+
+/**
+ * Called repeatedly by epd_display_stream() to fill up to chunk_size bytes
+ * into chunk. Must return the number of bytes written, or 0 once exhausted.
+ */
+typedef size_t (*epd_read_fn_t)(uint8_t *chunk, size_t chunk_size, void *ctx);
+
+/**
+ * Streams a full frame (EPD_FRAME_BYTES bytes, packed 2 pixels/byte) to the
+ * panel via read_fn and refreshes. Pulling from a caller-supplied source
+ * instead of a single buffer lets callers feed the panel directly from an
+ * HTTP response without holding the whole ~192KB frame in RAM.
+ */
+esp_err_t epd_display_stream(epd_read_fn_t read_fn, void *ctx);
+
+/** Convenience wrapper around epd_display_stream() for an in-memory frame buffer. */
+esp_err_t epd_display_buffer(const uint8_t *frame, size_t len);
+
+/** Puts the panel into deep sleep to minimize power draw between refreshes. */
+esp_err_t epd_sleep(void);