Merge mcarper/keymindCascade into mqtt-observer-plus

Integrates Mike Carper's transmission-reliability work (direct retries with
SNR-adaptive backoff and CR, flood retry controls, alt-path replies, flood
channel gates) plus its upstream/dev base (as of 2026-07-06) on top of the
observer stack. The flex branch is untouched; this branch is the experimental
integration line.

Conflict-resolution decisions, for future re-syncs (git rerere is enabled and
has recorded these):
- NodePrefs adopts keymind/upstream member order + retry/flood tail. Member
  order is in-memory only: /com_prefs stays field-by-field with the same
  canonical file order both sides already share through offset 294; keymind
  appends the retry tail at 295+ (new canonical size 676). Flex fleet files
  load unchanged; retry fields default via direct_retry_prefs_magic.
- loadPrefsInt keeps the fork's legacy MQTT-gap recovery but moves its
  detection boundary from 'extra > 5' to 'extra > LEGACY_MQTT_GAP_3SLOT (864)'
  so keymind-size tails take the normal read path. COM_PREFS_TAIL_BYTES
  retired.
- Dispatcher/RadioLibWrappers: fork watchdog additions + keymind TX overrides
  and CAD busy counter are unioned; RxReservePacketManager::queueOutbound
  follows upstream's new bool return (false when shedding).
- CommonCLI: fork observer dispatch + versioned /mqtt_prefs machinery kept;
  keymind CLI (retry/flood/radioat) taken; duplicate CAD/FEM handlers and
  sanitise lines deduped (kept keymind placements to minimize future diff).
- MyMesh (repeater/room): fork alerter/OTA/observer wiring kept; keymind
  scheduled-radio system replaces the old pending_* temp-radio members;
  applySavedRadioParams() replaces raw setParams at startup.
- ESP32Board: fork manifest-OTA + keymind powerOff/deepSleep and stopOTAUpdate
  unioned; startOTAUpdate keeps fork's STA-aware IP with keymind's idempotency
  guard.
- build.sh taken wholesale from keymind (fleet builds use GH Actions).

Verified: Heltec_v3_repeater_observer_mqtt, Heltec_v3_repeater,
heltec_v4_repeater_observer_mqtt, Heltec_v3_room_server_observer_mqtt build;
host migration harness (13 MQTT + non-MQTT scenarios) passes with updated
expectations (676-byte /com_prefs; odd-size files normalize on save, not
load); native unit tests 13/13.

examples/simple_secure_chat/main.cpp

@@ -135,7 +135,7 @@ class MyMesh : public BaseChatMesh, ContactVisitor {
     File file = _fs->open("/contacts", "w", true);
 #endif
     if (file) {
-      ContactsIterator iter;
+      ContactsIterator iter = startContactsIterator();
       ContactInfo c;
       uint8_t unused = 0;
       uint32_t reserved = 0;
@@ -202,6 +202,10 @@ protected:
     return true;
   }
 
+  uint8_t getDefaultTxCodingRate() const override {
+    return LORA_CR;
+  }
+
   void onDiscoveredContact(ContactInfo& contact, bool is_new, uint8_t path_len, const uint8_t* path) override {
     // TODO: if not in favs,  prompt to add as fav(?)