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

test/test_nmea/test_nmea.cpp

@@ -2,6 +2,7 @@
 
 #include <cstdio>
 #include <string>
+#include <vector>
 
 #include "nmea_parser.h"
 
@@ -195,6 +196,17 @@ void test_an_overlong_line_is_dropped() {
     TEST_ASSERT_TRUE(p.state().positionValid);
 }
 
+void test_every_complete_line_reaches_the_echo() {
+    NmeaParser p;
+    std::vector<std::string> seen;
+    p.onLine = [&](const std::string& l) { seen.push_back(l); };
+    std::string data = "$GNVTG,,,,,,,,,N*2E\r\nbroken\r\n$GN";
+    p.feed(data.data(), data.size());
+    TEST_ASSERT_EQUAL(2, static_cast<int>(seen.size()));
+    TEST_ASSERT_EQUAL_STRING("$GNVTG,,,,,,,,,N*2E", seen[0].c_str());
+    TEST_ASSERT_EQUAL_STRING("broken", seen[1].c_str());
+}
+
 void test_constellation_names() {
     TEST_ASSERT_EQUAL_STRING("GPS", constellationName(Constellation::Gps));
     TEST_ASSERT_EQUAL_STRING("Galileo", constellationName(Constellation::Galileo));
@@ -218,6 +230,7 @@ int main() {
     RUN_TEST(test_gsa_marks_the_satellites_used_per_system);
     RUN_TEST(test_split_feeds_and_noise_are_handled);
     RUN_TEST(test_an_overlong_line_is_dropped);
+    RUN_TEST(test_every_complete_line_reaches_the_echo);
     RUN_TEST(test_constellation_names);
     return UNITY_END();
 }