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/gnss/src/nmea_parser.cpp

@@ -92,7 +92,10 @@ void NmeaParser::feed(const char* data, size_t len) {
     for (size_t i = 0; i < len; i++) {
         char c = data[i];
         if (c == '\n') {
-            if (!overflow_ && !line_.empty()) sentence(line_);
+            if (!overflow_ && !line_.empty()) {
+                if (onLine) onLine(line_);
+                sentence(line_);
+            }
             line_.clear();
             overflow_ = false;
         } else if (c != '\r') {