Add command-skeleton logging for direct comparison against a captured epaper_spi trace

Sixth hardware run (10MHz SPI clock) still solid black. Rather than keep
guessing individual protocol bytes, log every command+data byte we send
(skipping bulk plane data - just a one-line summary per plane write) so
this run's trace can be diffed directly against a captured trace of the
known-working epaper_spi driver on the same physical panel.

components/gxepd2/cycle_engine.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <cstdint>
+#include <cstdio>
 
 #include "esphome/core/gpio.h"
 #include "esphome/core/hal.h"
@@ -168,6 +169,19 @@ class CycleEngine {
   }
 
   void write_command_(uint8_t cmd, const uint8_t *data, uint8_t data_len) {
+#if ESPHOME_LOG_LEVEL >= ESPHOME_LOG_LEVEL_DEBUG
+    // Command-level skeleton trace - deliberately left in (not gated to
+    // VERY_VERBOSE) while diagnosing a real-hardware content bug: this is
+    // the exact byte-for-byte counterpart to compare against a captured
+    // epaper_spi trace on the same panel. Bulk plane data isn't logged
+    // here (see step_write_plane_) - only the command skeleton matters for
+    // that comparison.
+    char buf[32];
+    int n = 0;
+    for (uint8_t i = 0; i < data_len && n < (int) sizeof(buf) - 3; i++)
+      n += snprintf(buf + n, sizeof(buf) - n, "%02X ", data[i]);
+    ESP_LOGD("gxepd2", "cmd 0x%02X data[%u]: %s", cmd, data_len, buf);
+#endif
     this->write_byte_(cmd, false);
     for (uint8_t i = 0; i < data_len; i++)
       this->write_byte_(data[i], true);
@@ -227,8 +241,12 @@ class CycleEngine {
   static constexpr uint32_t BYTE_BUDGET = 64;
 
   void step_write_plane_() {
-    if (this->cursor_ == 0)
+    if (this->cursor_ == 0) {
+      ESP_LOGD("gxepd2", "plane %u: cmd 0x%02X, %u bytes, invert=%d", this->plane_,
+               this->desc_->plane_write_cmd[this->plane_], (unsigned) this->plane_bytes_,
+               this->desc_->plane_invert[this->plane_]);
       this->write_byte_(this->desc_->plane_write_cmd[this->plane_], false);
+    }
     const uint8_t *buf = this->planes_[this->plane_];
     bool invert = this->desc_->plane_invert[this->plane_];
     uint32_t end = this->cursor_ + BYTE_BUDGET;