M2 step 3: GNSS Service, with standby and the clock from a Fix

The GNSS Service reads the receiver (UART 15/13, 115200, 1 KB buffer)
from its 50 ms tick and feeds the NMEA parser. With a Fix it sets the
clock (GNSS is the most trusted TimeSource) and refreshes it every 10
min. Settings gets GNSS On/Off and the coordinate format (Q64).

Off puts the receiver in standby with $PCAS12,65535, renewed hourly; On
wakes it with a hot start, $PCAS10,0. Both measured on the device: the
output stops within a second, and a command wakes it within a second.

Commands: gnss status (Fix, satellites per constellation, bytes and
sentences), gnss nmea on|off, gnss send <sentence>, gnss restart. The
probe is gone; never drive GPIO 15 (the receiver's output): the first
probe's swapped-pin attempt silenced it until a power cycle.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/services/src/settings.cpp

@@ -29,6 +29,8 @@ const Definition kDefinitions[] = {
     {"sound", Kind::Bool, 1, nullptr, 0, 1},
     {"probe_mac_raw", Kind::Bool, 1, nullptr, 0, 1},
     {"wifi_on", Kind::Bool, 1, nullptr, 0, 1},
+    {"gnss_on", Kind::Bool, 1, nullptr, 0, 1},
+    {"coord_dms", Kind::Bool, 0, nullptr, 0, 1},
 };
 static_assert(sizeof(kDefinitions) / sizeof(kDefinitions[0]) == static_cast<size_t>(Setting::Count),
               "every Setting needs a definition");