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

src/services/gnss_service.h

@@ -0,0 +1,65 @@
+#pragma once
+
+#include <Print.h>
+
+#include <string>
+
+#include "event_bus.h"
+#include "nmea_parser.h"
+#include "service.h"
+#include "services/clock_service.h"
+#include "settings.h"
+
+namespace roro {
+
+// The GNSS receiver on the Cap LoRa-1262 (see CONTEXT.md and docs/milestones/M2.md): reads its
+// NMEA from the main loop's tick (about 450 bytes/s, so a 1 KB UART buffer covers any stall),
+// holds the current Fix, and sets the clock from it. Settings → GNSS switches it on and off.
+class GnssService : public Service {
+   public:
+    static constexpr int kRxPin = 15, kTxPin = 13;  // measured: M2 step 1
+    static constexpr uint32_t kBaud = 115200;
+    static constexpr uint32_t kSilentAfterMs = 3000;  // no sentence for this long: receiver silent
+    static constexpr uint32_t kClockEveryMs = 600000; // refresh the clock from the Fix this often
+    // Off (Q58, measured): "$PCAS12,<s>" puts the receiver in standby for that long, and any
+    // command wakes it. Standby is renewed while off, in case the duration is clamped.
+    static constexpr uint32_t kStandbyRenewMs = 3600000;
+
+    GnssService(const Settings& settings, ClockService& clock, EventBus& bus);
+    const char* name() const override { return "gnss"; }
+    uint32_t tickIntervalMs() const override { return 50; }
+    void start() override;
+    void stop() override;
+    void tick(uint32_t nowMs) override;
+
+    bool on() const { return active_; }
+    bool receiving(uint32_t nowMs) const { return active_ && lastLineMs_ && nowMs - lastLineMs_ < kSilentAfterMs; }
+    const gnss::GnssState& state() const { return parser_.state(); }
+
+    // Debug aids (Q68): raw sentences to the console, a status report, and sending a command
+    // ("PCAS10,0" becomes "$PCAS10,0*1C\r\n").
+    void setEcho(bool echo) { echo_ = echo; }
+    void printStatus(Print& out, uint32_t nowMs) const;
+    bool send(const std::string& body);
+    void restart(uint32_t nowMs) {
+        close();
+        open(nowMs);
+    }
+
+   private:
+    void open(uint32_t nowMs);  // the UART; the receiver's state follows the setting in tick()
+    void close();
+    void wake(uint32_t nowMs);
+    void standby(uint32_t nowMs);
+
+    const Settings& settings_;
+    ClockService& clock_;
+    EventBus& bus_;
+    gnss::NmeaParser parser_;
+    bool open_ = false, active_ = false, echo_ = false;
+    uint32_t bytes_ = 0, standbyMs_ = 0;
+    uint32_t openedMs_ = 0, lastLineMs_ = 0, firstFixMs_ = 0, clockSetMs_ = 0;
+    gnss::FixType lastFix_ = gnss::FixType::None;
+};
+
+}  // namespace roro