fix(wifi): keep the legacy power-save default off; store explicit min as 3

Stored 0 was the shipped default from 2026-01-02 to 2026-03-28, and every
association path has run it with power save off since. Reading it as
MIN_MODEM, as F11 did, would have put every node set up in that window
into modem sleep on its next association. 0 now reads as `none`, and
`set wifi.powersave min` stores a new value 3. /mqtt.json accepts 0..3;
older firmware repairs 3 to `none` on load. Binary snapshots are no longer
written, so 3 never reaches a legacy layout.

src/helpers/MQTTPrefsSerializer.h

@@ -4,6 +4,7 @@
 #include <helpers/MQTTObserverValidation.h>
 #include <helpers/MQTTPresets.h>
 #include <helpers/MQTTPrefsStorage.h>
+#include <helpers/WifiPowerSavePolicy.h>
 
 #ifdef WITH_MQTT_BRIDGE
 
@@ -50,7 +51,7 @@ class MQTTPrefsSerializer : public ConfigSerializer {
   public:
     explicit WifiPrefs(MQTTPrefs* prefs) : _prefs(prefs), _power_save(prefs->wifi_power_save) {}
     bool apply(bool* repaired) {
-      if (_power_save < 0 || _power_save > 2) {
+      if (_power_save < 0 || _power_save > WifiPowerSavePolicy::kMaxStoredValue) {
         _power_save = 1;
         *repaired = true;
       }

src/helpers/WifiPowerSavePolicy.h

@@ -11,15 +11,18 @@
 // to `min` silently ran with power save off after its first reconnect while
 // `get wifi.powersave` still said min.
 //
-// Stored values are fleet state — never renumber them. The product default is
-// `none`, which is a *default* (MQTTDefaults.h), not a reinterpretation of an
-// operator's explicit `min`.
+// Stored values are fleet state — never renumber them. 0 was the shipped
+// default from 2026-01-02 to 2026-03-28, and every association path ran it with
+// power save off, so it keeps meaning `none`; an explicit `min` is stored as 3.
+// Older firmware repairs 3 to `none` when it loads /mqtt.json.
 namespace WifiPowerSavePolicy {
 
 enum StoredValue : uint8_t {
-  kMin  = 0,   // WIFI_PS_MIN_MODEM
-  kNone = 1,   // WIFI_PS_NONE  (default)
-  kMax  = 2,   // WIFI_PS_MAX_MODEM
+  kLegacyDefault = 0,   // WIFI_PS_NONE: the old default, never an operator choice
+  kNone = 1,            // WIFI_PS_NONE  (default)
+  kMax  = 2,            // WIFI_PS_MAX_MODEM
+  kMin  = 3,            // WIFI_PS_MIN_MODEM
+  kMaxStoredValue = kMin,
 };
 
 // Mirrors wifi_ps_type_t. MQTTBridge.cpp static_asserts these against the SDK.

test/test_wifi_power_save_policy/test_wifi_power_save_policy.cpp

@@ -13,8 +13,18 @@ TEST(WifiPowerSavePolicy, StoredValuesMapToOneModeEach) {
   EXPECT_EQ(kModeMaxModem, modeFor(kMax));
 }
 
+// Stored 0 was the default for nodes set up 2026-01-02..03-28, and they have
+// always run with power save off; reading it as `min` would put them to sleep.
+TEST(WifiPowerSavePolicy, LegacyDefaultKeepsPowerSaveOff) {
+  EXPECT_EQ(kModeNone, modeFor(kLegacyDefault));
+  EXPECT_STREQ("none", nameFor(kLegacyDefault));
+  uint8_t parsed = 0xFF;
+  ASSERT_TRUE(parseName("min", &parsed));
+  EXPECT_NE(kLegacyDefault, parsed);
+}
+
 TEST(WifiPowerSavePolicy, NamesRoundTripWithStoredValues) {
-  for (uint8_t stored = 0; stored <= 2; stored++) {
+  for (uint8_t stored = kNone; stored <= kMaxStoredValue; stored++) {
     uint8_t parsed = 0xFF;
     ASSERT_TRUE(parseName(nameFor(stored), &parsed)) << "stored " << (int)stored;
     EXPECT_EQ(stored, parsed);
@@ -25,7 +35,7 @@ TEST(WifiPowerSavePolicy, NamesRoundTripWithStoredValues) {
 // A byte outside the stored range must read as the product default, not as
 // whatever mode happens to sit at that index.
 TEST(WifiPowerSavePolicy, OutOfRangeStoredValueReadsAsDefault) {
-  for (int stored = 3; stored <= 255; stored++) {
+  for (int stored = kMaxStoredValue + 1; stored <= 255; stored++) {
     EXPECT_EQ(kModeNone, modeFor((uint8_t)stored)) << "stored " << stored;
     EXPECT_STREQ("none", nameFor((uint8_t)stored));
   }