feat(prefs): tail schema sentinel for layout-change detection

Old saves (without sentinel) load unchanged — rd() lambda zero-inits
anything not on disk. New saves append a 4-byte sentinel that encodes
the schema revision; the low byte is bumped on every layout change in
loadPrefsInt/savePrefs. Mismatch on load logs a warning and continues
with defaults; the next savePrefs writes the current sentinel so the
file self-heals.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/DataStore.cpp

@@ -304,6 +304,16 @@ void DataStore::loadPrefsInt(const char *filename, NodePrefs& _prefs, double& no
     _prefs.page_order_set = NodePrefs::PAGE_ORDER_MAGIC;
   }
 
+  // Schema sentinel: bumped on layout changes. Mismatch means an older file
+  // (or a different schema); rd() already zero-inits any fields not present,
+  // so we just log it — next savePrefs writes the current sentinel.
+  uint32_t sentinel = 0;
+  rd(&sentinel, sizeof(sentinel));
+  if (sentinel != NodePrefs::SCHEMA_SENTINEL) {
+    MESH_DEBUG_PRINTLN("prefs schema sentinel mismatch: got 0x%08X, expected 0x%08X — re-saving on next change",
+                       (unsigned)sentinel, (unsigned)NodePrefs::SCHEMA_SENTINEL);
+  }
+
   file.close();
 }
 
@@ -385,6 +395,10 @@ void DataStore::savePrefs(const NodePrefs& _prefs, double node_lat, double node_
     file.write((uint8_t *)&_prefs.eink_full_refresh_every, sizeof(_prefs.eink_full_refresh_every));
     file.write((uint8_t *)&_prefs.page_order_set, sizeof(_prefs.page_order_set));
 
+    // Tail sentinel — must be last. See NodePrefs::SCHEMA_SENTINEL.
+    uint32_t sentinel = NodePrefs::SCHEMA_SENTINEL;
+    file.write((uint8_t *)&sentinel, sizeof(sentinel));
+
     file.close();
   }
 }