Arrival time fix perhaps (#11274)

* Add explicit presence for MeshPacket.rx_time (arrival time)

rx_time is now proto3 optional with a has_rx_time presence bit, matching
the rx_rssi treatment. A node with no GPS and no phone connected yet has
no time source at all, so a bare 0 was indistinguishable from a genuine
1970-01-01 reading; downstream consumers (replay packets, JSON
serialization) now check has_rx_time instead of the value.

* Dedupe rx_time stamping into a shared helper; trim a debug log string

Extract the repeated haveTime/rx_time/has_rx_time stamp logic (5 call
sites across Router.cpp, MeshBeaconModule.cpp, MeshService.cpp) into
Router::computeRxTimeStamp()/stampRxTime(). Also shorten the new RTC.cpp
LOG_DEBUG string. Saves 48 bytes of flash on rak4631 (measured), no
behavior change.

* Fix has_rx_rssi presence carried unconditionally through StoreForward replay

preparePayload() set has_rx_rssi = true unconditionally on replay, regardless
of whether the packet's rx_rssi at store time was a genuine measurement (e.g.
MQTT-relayed packets carry no real RSSI). Store the presence bit alongside
rx_rssi in PacketHistoryStruct and restore it on replay instead.

Flagged by Copilot on #11271 (same root cause the has_rx_time explicit
presence work fixes) but never addressed before that PR merged.

* Trim comment blocks to the repo's 1-2 line guideline

.github/copilot-instructions.md:338 caps code comments at 1-2 lines; several
blocks added across the rx_time explicit-presence work ran well past that.
Also consolidates Time.cpp's file-level doc comment into Time.h, where the
rest of the Time:: API contract already lives.

No behavior change.

* Add rx_time explicit-presence test coverage

- test_meshpacket_serializer: has_rx_time=false fixture plus tests asserting
  JsonSerialize/JsonSerializeEncrypted emit 0 rather than leaking the
  millis() placeholder, alongside the has_rx_time=true baseline.
- test_stream_api: two tests driving a real PhoneAPI handshake (want_config_id
  through STATE_SEND_PACKETS) that simulate a phone time-giving transaction
  arriving before vs. after a queued packet is drained - covering both the
  reconciled and the ships-with-placeholder-absent paths of
  MeshService::reconcilePendingRxTimes().

* Fix three correctness issues flagged in review

- Time.h: drop the reserved-identifier include guard (_MT_TIME_H); pragma
  once already covers it, matching convention elsewhere (e.g. RTC.h).
- Time.cpp: rebase getMillis64()'s wrap accumulator when the test seam
  swaps clock sources, so a real<->injected clock jump isn't miscounted
  as a genuine 32-bit wrap.
- NodeInfoModule: the 12h reply-suppression window is a local dedup
  duration, not a wall-clock reading - switch it to Time::getMillis64()
  so RTC-quality jumps and replayed packets' stale rx_time can't perturb
  it.
- StoreForwardModule: has_rx_time was derived from *current* RTC quality
  at replay time rather than stored at capture time, so a history entry
  saved while time-blind could be misreported as a valid epoch once the
  clock later improved. Persist the presence bit in PacketHistoryStruct
  instead.

* tryfix CI

* post review fixes

* more test fixes

test/test_meshpacket_serializer/test_helpers.h

@@ -37,6 +37,7 @@ static meshtastic_MeshPacket create_test_packet(meshtastic_PortNum port, const u
     packet.want_ack = false;
     packet.priority = meshtastic_MeshPacket_Priority_UNSET;
     packet.rx_time = 1609459200;
+    packet.has_rx_time = true; // rx_time has explicit presence; mark this synthetic reading as measured
     packet.rx_snr = 10.5f;
     packet.hop_start = 3;
     packet.rx_rssi = -85;
@@ -62,3 +63,14 @@ static meshtastic_MeshPacket create_test_packet(meshtastic_PortNum port, const u
 
     return packet;
 }
+
+// Same as create_test_packet(), but with rx_time absent (has_rx_time = false) and a nonzero
+// millis()-style placeholder in rx_time, to verify serializers treat it as unknown, not a reading.
+static meshtastic_MeshPacket create_test_packet_no_rx_time(meshtastic_PortNum port, const uint8_t *payload, size_t payload_size,
+                                                           int payload_variant = meshtastic_MeshPacket_decoded_tag)
+{
+    meshtastic_MeshPacket packet = create_test_packet(port, payload, payload_size, payload_variant);
+    packet.rx_time = 123456; // a plausible millis() placeholder, not a real epoch
+    packet.has_rx_time = false;
+    return packet;
+}