fix(companion): channel save/lookup hardening + trail count guard

Address the open items from the FEATURES.md audit backlog:
- saveChannels() now skips unused slots (all-zero secret) instead of
  writing all 40 every time, so /channels2 holds only configured channels
  (was always ~2.7 KB) and wears the flash less. loadChannels() already
  compacted empties on read, so the loaded result is unchanged.
- findChannelIdx() returns -1 for an all-zero secret, so a corrupted/empty
  channel can't match an unused all-zero slot and misroute messages.
- TrailStore gains a static_assert that CAPACITY fits the uint16_t save
  header count, failing the build instead of silently truncating.

Also re-classify two audit items verified to be non-issues in current
code: the bot strstr "199-char" truncation (BOT_SCRATCH=200 >= MAX_TEXT_LEN
=160, unreachable) and the MSG_PICK reply title (rlen clamped to 20, so
title[24] never overflows).

Verified: WioTrackerL1_companion_solo_dual builds clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/DataStore.cpp

@@ -685,12 +685,19 @@ void DataStore::saveChannels(DataStoreHost* host) {
     memset(unused, 0, 4);
 
     while (host->getChannelForSave(channel_idx, ch)) {
+      channel_idx++;
+      // getChannelForSave() returns every slot up to MAX_GROUP_CHANNELS, so skip
+      // the unused ones (all-zero secret) rather than writing all 40 — otherwise
+      // the file is always ~2.7 KB and wears the flash needlessly. loadChannels()
+      // already compacts empty entries on read, so the loaded result is identical.
+      bool empty = true;
+      for (int b = 0; b < 32; b++) if (ch.channel.secret[b]) { empty = false; break; }
+      if (empty) continue;
+
       bool success = (file.write(unused, 4) == 4);
       success = success && (file.write((uint8_t *)ch.name, 32) == 32);
       success = success && (file.write((uint8_t *)ch.channel.secret, 32) == 32);
-
       if (!success) break; // write failed
-      channel_idx++;
     }
     file.close();
   }