Add explicit presence for arrival time: MeshPacket.rx_time optional

[NomDeTom]

Needed for meshtastic/firmware#11274

Summary by CodeRabbit

• Bug Fixes
  • Packet reception timestamps can now be explicitly marked as unavailable when a reliable clock source is not present.
  • Improved distinction between an unknown timestamp and a valid Unix epoch timestamp.

[coderabbitai]

📝 Walkthrough

Walkthrough

MeshPacket.rx_time is now an explicitly optional protobuf field, with documentation describing timestamp absence when no reliable wall clock is available.

Changes

MeshPacket timestamp contract

Layer / File(s)	Summary
Optional reception timestamp contract meshtastic/mesh.proto	MeshPacket.rx_time uses proto3 optional presence semantics, and its documentation distinguishes absent timestamps from Unix epoch values and allows later re-timestamping.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• meshtastic/protobufs#1024: Updates MeshPacket receiver telemetry with similar proto3 optional presence semantics.

Suggested reviewers: thebentern, jp-bennett, caveman99

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is far too minimal and does not follow the required template sections or checklist.	Add the required 'What does this PR do?' section, replace the placeholder issue link if needed, and include the checklist items.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly states the main change: making MeshPacket.rx_time optional for explicit presence.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[NomDeTom]

@jamesarich another minor change.