feat(ui): channel "relayed into mesh" marker (✓ on repeater echo)

Channels have no recipient ACK, so true delivery can't be shown. Instead reuse
the heard-repeater-echo idea (same as APC's flood feedback) to mark an outgoing
channel message ✓ once a repeater rebroadcast of it is heard — i.e. it was
relayed into the mesh. No echo is NOT a failure (direct/0-hop neighbours never
echo), so unconfirmed sends simply show no marker.

- MyMesh: single-slot relay tracker independent of APC (trackRelaySend on every
  channel flood; hash-match in filterRecvFloodPacket gated on _relay_pending so
  the hot recv path is untouched otherwise; window expiry in loop just drops it).
  lastChannelRelaySeq() hands the seq to the UI.
- AbstractUITask::onChannelRelayed() -> UITask -> QuickMsgScreen::markChannelRelayed().
- ChHistEntry gains relay_status / relay_seq; afterSend arms the marker on the
  exact stored entry; render shows ✓ only on ACK_OK for our own ("Me:") rows.

Verified: GAT562 (SSD1306) and WioTrackerL1Eink (GxEPD) solo builds compile clean.

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

examples/companion_radio/MyMesh.h

@@ -201,6 +201,12 @@ protected:
   void apcSampleSnr(float snr);
   void apcOnFailure();
   void apcTrackFloodSend(const mesh::Packet* pkt);
+  void trackRelaySend(const mesh::Packet* pkt);   // arm the UI relayed-into-mesh tracker
+public:
+  // Seq of the most recently tracked channel send — the UI records it on the
+  // outgoing history entry so a heard echo (onChannelRelayed) can match it back.
+  uint32_t lastChannelRelaySeq() const { return _last_relay_seq; }
+private:
 
   // DataStoreHost methods
   bool onContactLoaded(const ContactInfo& contact) override { return addContact(contact); }
@@ -291,6 +297,15 @@ private:
   uint16_t _apc_flood_len;                 // its payload length — cheap pre-filter before hashing
   uint32_t _apc_flood_deadline;            // echo-wait deadline for that send
   bool _apc_flood_pending;                 // a tracked flood send is awaiting its echo
+  // UI "relayed into mesh" tracker for channel sends — independent of APC, single
+  // slot (only the latest channel send is tracked). Hashing on receive only runs
+  // while a send is pending, so the hot flood-recv path is untouched otherwise.
+  uint8_t  _relay_hash[MAX_HASH_SIZE];
+  uint16_t _relay_len;
+  uint32_t _relay_deadline;
+  uint32_t _relay_seq;        // monotonic id of the pending tracked send
+  uint32_t _last_relay_seq;   // seq of the most recent tracked send (for the UI to record)
+  bool     _relay_pending;
   bool send_unscoped;   // force un-scoped flood (instead of using send_scope)
   char cli_command[80];
   uint8_t app_target_ver;