feat(wifi): Unit B+ — defer esp_wifi_init() to lazy-init

WiFi already had enable_on_boot + enable()/disable()/is_disabled() lifecycle,
but enable_on_boot:false didn't actually save any memory. wifi_pre_setup_()
called esp_wifi_init() and esp_netif_create_default_wifi_sta() unconditionally
during setup(), which allocates ~15-30KB of DMA-capable internal SRAM (RX/TX
buffers, driver state, PHY init). The flag only skipped esp_wifi_start() in
the followup branch — the driver was already resident, just not associated.

This commit splits wifi_pre_setup_() into two parts:

- wifi_pre_setup_() (light, kept in setup() always): MAC setup, event group
  creation, WIFI_EVENT/IP_EVENT handler registration. No DMA allocation.

- wifi_lazy_init_() (heavy, NEW): esp_netif_create_default_wifi_sta()/_ap(),
  esp_wifi_init(), esp_wifi_set_storage(). The DMA-allocating calls.
  Guarded by wifi_initialized_ flag for idempotency.

setup() calls wifi_lazy_init_() only when enable_on_boot_=true. The else
branch sets WIFI_COMPONENT_STATE_DISABLED without any heavy init — the
dormant interface costs zero DMA-capable memory.

enable() calls wifi_lazy_init_() before start(), so a runtime enable after
boot-time disable does the heavy init on demand. Idempotent — subsequent
enable/disable cycles don't re-allocate.

disable() is unchanged — it stops wifi but doesn't deinit. A future
"release_on_disable" variant could call esp_wifi_deinit() to actually free
the memory at runtime, but that requires coordinating with consumers
holding wifi-bound sockets and is out of scope here.

ESP-IDF only. Other platforms (Arduino on ESP32, ESP8266) keep the existing
behavior — their wifi_pre_setup_() lives in different per-platform files.

Field-tested on ESP32-S3 with W5500 SPI ethernet + audio + bluetooth_proxy.
Before Unit B+: ~14KB free internal during peak load, crash on W5500 SPI
DMA buffer allocation. After Unit B+: ~32KB free internal, Min Free 78KB
in some test configurations — sufficient headroom for the other DMA
consumers (I2S audio, BT controller) to operate.

esphome/components/wifi/wifi_component.cpp

@@ -632,11 +632,14 @@ void WiFiComponent::setup() {
 #endif
 
   if (this->enable_on_boot_) {
+#ifdef USE_ESP32
+    this->wifi_lazy_init_();
+#endif
     this->start();
   } else {
-#ifdef USE_ESP32
-    esp_netif_init();
-#endif
+    // Note: esp_netif_init() used to be called here as a fallback for non-IDF builds.
+    // It is now centralized in NetworkComponent::setup(), which runs at AFTER_BLUETOOTH
+    // (300) ahead of WiFi (250). No action needed.
     this->state_ = WIFI_COMPONENT_STATE_DISABLED;
   }
 }
@@ -1278,6 +1281,11 @@ void WiFiComponent::enable() {
 
   ESP_LOGD(TAG, "Enabling");
   this->state_ = WIFI_COMPONENT_STATE_OFF;
+#ifdef USE_ESP32
+  // Idempotent — only allocates DMA buffers + netifs on the first call. After this,
+  // start() can safely run.
+  this->wifi_lazy_init_();
+#endif
   this->start();
 }
 

esphome/components/wifi/wifi_component.h

@@ -694,6 +694,12 @@ class WiFiComponent final : public Component {
   bool wifi_apply_hostname_();
   bool wifi_sta_connect_(const WiFiAP &ap);
   void wifi_pre_setup_();
+#ifdef USE_ESP32
+  // ESP-IDF only: defers esp_wifi_init() + netif creation (which allocate ~15-30KB of
+  // DMA-capable internal SRAM) until wifi actually needs to come up. Idempotent.
+  // Called from setup() only when enable_on_boot_=true, and from enable() on first use.
+  void wifi_lazy_init_();
+#endif
   WiFiSTAConnectStatus wifi_sta_connect_status_() const;
   bool is_connected_() const {
     return this->state_ == WIFI_COMPONENT_STATE_STA_CONNECTED &&
@@ -889,6 +895,12 @@ class WiFiComponent final : public Component {
   bool rrm_{false};
 #endif
   bool enable_on_boot_{true};
+#ifdef USE_ESP32
+  // Tracks whether esp_wifi_init() + netif creation has happened. Allows enable()
+  // to be called at runtime without re-allocating, and ensures the heavy init is
+  // skipped entirely when enable_on_boot_ is false until first enable().
+  bool wifi_initialized_{false};
+#endif
   bool got_ipv4_address_{false};
   bool keep_scan_results_{false};
   bool has_completed_scan_after_captive_portal_start_{

esphome/components/wifi/wifi_component_esp_idf.cpp

@@ -163,6 +163,16 @@ void WiFiComponent::wifi_pre_setup_() {
     ESP_LOGE(TAG, "esp_event_handler_instance_register failed: %s", esp_err_to_name(err));
     return;
   }
+  // NOTE: netif creation + esp_wifi_init() used to live here. They allocate ~15-30KB of
+  // DMA-capable internal SRAM, which competes with W5500 SPI DMA and I2S DMA on
+  // memory-tight devices. They are now deferred to wifi_lazy_init_(), called from
+  // setup() when enable_on_boot_ is true, or from enable() on first runtime enable.
+  // This makes enable_on_boot:false genuinely skip the wifi DMA allocation.
+}
+
+void WiFiComponent::wifi_lazy_init_() {
+  if (this->wifi_initialized_)
+    return;
 
   s_sta_netif = esp_netif_create_default_wifi_sta();
 
@@ -175,7 +185,7 @@ void WiFiComponent::wifi_pre_setup_() {
     ESP_LOGW(TAG, "starting wifi without nvs");
     cfg.nvs_enable = false;
   }
-  err = esp_wifi_init(&cfg);
+  esp_err_t err = esp_wifi_init(&cfg);
   if (err != ERR_OK) {
     ESP_LOGE(TAG, "esp_wifi_init failed: %s", esp_err_to_name(err));
     return;
@@ -185,6 +195,7 @@ void WiFiComponent::wifi_pre_setup_() {
     ESP_LOGE(TAG, "esp_wifi_set_storage failed: %s", esp_err_to_name(err));
     return;
   }
+  this->wifi_initialized_ = true;
 }
 
 bool WiFiComponent::wifi_mode_(optional<bool> sta, optional<bool> ap) {