[esp8266] Wrap printf/vprintf/fprintf to eliminate _vfprintf_r (~900 bytes flash)

Apply the same linker wrap technique used on ESP32 (PR #14362) to ESP8266.
ESPHome logging uses ets_printf, not libc printf, so the FILE*-based printf
path is dead code. The stubs redirect through vsnprintf + fwrite, allowing
the linker to GC _vfprintf_r.

Savings are smaller than ESP32 (~900 bytes vs ~11 KB) because ESP8266's
newlib printf is more modular, but ESP8266 has much less flash headroom
so every byte counts.

esphome/components/esp8266/__init__.py

@@ -260,6 +260,11 @@ async def to_code(config):
     if CORE.testing_mode:
         cg.add_build_flag("-DESPHOME_TESTING_MODE")
 
+    # Wrap FILE*-based printf functions to eliminate newlib's _vfprintf_r
+    # (~900 bytes). See printf_stubs.cpp for implementation.
+    for symbol in ("vprintf", "printf", "fprintf"):
+        cg.add_build_flag(f"-Wl,--wrap={symbol}")
+
     cg.add_platformio_option("board_build.flash_mode", config[CONF_BOARD_FLASH_MODE])
 
     ver: cv.Version = CORE.data[KEY_CORE][KEY_FRAMEWORK_VERSION]

esphome/components/esp8266/printf_stubs.cpp

@@ -0,0 +1,71 @@
+/*
+ * Linker wrap stubs for FILE*-based printf functions.
+ *
+ * The ESP8266 Arduino framework and libraries may reference printf(),
+ * vprintf(), and fprintf() which pull in newlib's _vfprintf_r (~900 bytes).
+ * ESPHome never uses these — all logging goes through ets_printf/ets_vsnprintf
+ * directly, so the libc FILE*-based printf path is dead code.
+ *
+ * These stubs redirect through vsnprintf() (which is already in the binary
+ * for ESPHome's logging) and fwrite(), allowing the linker to dead-code
+ * eliminate _vfprintf_r.
+ *
+ * Saves ~900 bytes of flash.
+ */
+
+#if defined(USE_ESP8266) && !defined(USE_FULL_PRINTF)
+#include <cstdarg>
+#include <cstdio>
+#include <cstdlib>
+
+namespace esphome::esp8266 {}
+
+static constexpr size_t PRINTF_BUFFER_SIZE = 128;
+
+// These stubs are essentially dead code at runtime — ESPHome uses ets_printf
+// for logging, and the Arduino core's Serial.printf() has its own implementation.
+// The buffer overflow check is purely defensive and should never trigger.
+static int write_printf_buffer(FILE *stream, char *buf, int len) {
+  if (len < 0) {
+    return len;
+  }
+  size_t write_len = len;
+  if (write_len >= PRINTF_BUFFER_SIZE) {
+    fwrite(buf, 1, PRINTF_BUFFER_SIZE - 1, stream);
+    abort();
+  }
+  if (fwrite(buf, 1, write_len, stream) < write_len || ferror(stream)) {
+    return -1;
+  }
+  return len;
+}
+
+// NOLINTBEGIN(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+extern "C" {
+
+int __wrap_vprintf(const char *fmt, va_list ap) {
+  char buf[PRINTF_BUFFER_SIZE];
+  return write_printf_buffer(stdout, buf, vsnprintf(buf, sizeof(buf), fmt, ap));
+}
+
+int __wrap_printf(const char *fmt, ...) {
+  va_list ap;
+  va_start(ap, fmt);
+  int len = __wrap_vprintf(fmt, ap);
+  va_end(ap);
+  return len;
+}
+
+int __wrap_fprintf(FILE *stream, const char *fmt, ...) {
+  va_list ap;
+  va_start(ap, fmt);
+  char buf[PRINTF_BUFFER_SIZE];
+  int len = write_printf_buffer(stream, buf, vsnprintf(buf, sizeof(buf), fmt, ap));
+  va_end(ap);
+  return len;
+}
+
+}  // extern "C"
+// NOLINTEND(bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,readability-identifier-naming)
+
+#endif  // USE_ESP8266 && !USE_FULL_PRINTF