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.h

@@ -2,6 +2,7 @@
 
 #include <cstddef>
 #include <cstdint>
+#include <functional>
 #include <map>
 #include <set>
 #include <string>
@@ -53,6 +54,9 @@ class NmeaParser {
     // One sentence without its line end. False if malformed or its checksum is wrong.
     bool sentence(const std::string& line);
 
+    // Called with every complete line fed in, valid or not (`gnss nmea on` echoes them).
+    std::function<void(const std::string&)> onLine;
+
     const GnssState& state() const { return state_; }
     uint32_t goodSentences() const { return good_; }
     uint32_t badSentences() const { return bad_; }