fix(mesh): coerce coordinate traffic to the position channel on event builds (#11545)

Under USERPREFS_BLOCK_POSITION_ON_EVENT_CHANNEL every coordinate packet a
client aimed at the event channel was rejected with the "Location sharing is
disabled on this channel" notification - including the phone's own location
feed. Both apps hand a GPS-less node its fix as a POSITION_APP packet
addressed to the node itself on channel 0; that packet never leaves the
device (Router::sendLocal delivers it locally) but resolved to the event
channel and was dropped before PositionModule saw it. Result: the toast on
every location tick, and nodes without a GPS never learned a position to
share on their private channel.

Position traffic now converges on the position channel - findPositionChannel(),
the first channel with non-zero on-wire precision, which is never the event
channel:

- From-us-to-us coordinate packets are exempt from the event block.
- Local coordinate sends aimed at the event channel (phone share-location,
  request-position, waypoints, any module/UI originator) are moved onto the
  position channel in Router::sendLocal and PhoneAPI instead of rejected. The
  client notification is only sent when no channel carries positions at all.
- A position request DM'd to us on the event channel is answered on the
  position channel at that channel's precision (request_id preserved, same
  reply throttle); the requester's coordinates are still not stored,
  forwarded, relayed or published. want_response from the bitfield is merged
  before the event-channel decode short-circuit so such requests are seen.
- PositionModule::sendOurPosition, positionUnchangedSinceLastSend and
  MeshService::trySendPosition use the shared helper instead of three copies
  of the same walk.

Non-event builds are unaffected: the coercion compiles out and the helper
matches the previous walk.

Tests: coverage-event-policy (test_event_channel_phone_api,
test_event_channel_router, test_position_precision, test_mqtt,
test_nexthop_routing) and the same suites with the policy off.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/mesh/PhoneAPI.cpp

@@ -1826,8 +1826,11 @@ bool PhoneAPI::handleToRadioPacket(meshtastic_MeshPacket &p)
     }
 #endif
 
-    // Reject before recording duplicate or per-port cooldown state, so a blocked
-    // attempt cannot throttle a valid private-channel position retry.
+    // Coordinates aimed at the event channel go out on the position channel instead (the phone picks the
+    // channel it last heard the node on, which is the event channel for everyone). Only when there is no
+    // channel to move them to is the send rejected. Reject before recording duplicate or per-port cooldown
+    // state, so a blocked attempt cannot throttle a valid private-channel position retry.
+    coerceCoordinatePacketToPositionChannel(&p);
     if (isBlockedEventCoordinatePacket(&p)) {
         LOG_DEBUG("Suppress phone coordinate send on event (everyone) channel");
         meshtastic_QueueStatus qs = router->getQueueStatus();