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

src/mesh/NodeDB.h

@@ -166,10 +166,15 @@ inline bool isRadioProfileFile(const char *filename)
            strcmp(filename, backupFileName) == 0;
 }
 
+/// "No trustworthy arrival time", as distinct from "zero seconds ago". Deliberately huge so the
+/// display formatters fall into their existing unknown-age branches ("unknown age" / "?").
+inline constexpr uint32_t SINCE_UNKNOWN = UINT32_MAX;
+
 /// Given a node, return how many seconds in the past (vs now) that we last heard from it
 uint32_t sinceLastSeen(const meshtastic_NodeInfoLite *n);
 
-/// Given a packet, return how many seconds in the past (vs now) it was received
+/// Given a packet, return how many seconds in the past (vs now) it was received,
+/// or SINCE_UNKNOWN if it carries no trustworthy rx_time.
 uint32_t sinceReceived(const meshtastic_MeshPacket *p);
 
 /// Outcome of mapping a single on-wire last-byte (next_hop / relay_node) back to a full NodeNum.