vehicle_time: Deliver VehicleClock callbacks from a worker thread

[florianfueller]

Improvement

Description

Implement callback delivery in VehicleClockBackendImpl; the Set*/Unset* methods were
no-ops so far.

• A worker thread, started by the first successful Init(), polls the TimeDaemon shared
  memory every 50 ms while at least one callback is registered and dispatches on that thread.
• TimeSlaveSyncData / PDelayMeasurementData fire on every changed frame.
• VehicleTimeStatus fires once after registration, then only when the flags change
  (rate deviation excluded).
• Subscribe / Unsubscribe are safe against in-flight invocations, including calls from
  inside a callback.
• Unit tests added; docs and comments updated, "not yet delivered" warnings removed.

Related ticket

closes #59 (improvement ticket)

[github-actions]

License Check Results

🚀 The license check job ran with the Bazel command:

bazel run //:license-check

Status: ⚠️ Needs Review

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[License Check Output]
Extracting Bazel installation...
Starting local Bazel server (8.6.0) and connecting to it...
INFO: Invocation ID: e47efd82-2490-4335-9f44-727f06ac8e2e
Computing main repo mapping: 
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WARNING: For repository 'platforms', the root module requires module version platforms@1.0.0, but got platforms@1.1.0 in the resolved dependency graph. Please update the version in your MODULE.bazel or set --check_direct_dependencies=off
WARNING: For repository 'score_platform', the root module requires module version score_platform@0.7.1, but got score_platform@0.7.2 in the resolved dependency graph. Please update the version in your MODULE.bazel or set --check_direct_dependencies=off
WARNING: For repository 'rules_oci', the root module requires module version rules_oci@2.2.7, but got rules_oci@2.3.0 in the resolved dependency graph. Please update the version in your MODULE.bazel or set --check_direct_dependencies=off
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Loading: 
Loading: 3 packages loaded
WARNING: Target pattern parsing failed.
ERROR: Skipping '//:license-check': no such target '//:license-check': target 'license-check' not declared in package '' defined by /home/runner/work/time/time/BUILD
ERROR: no such target '//:license-check': target 'license-check' not declared in package '' defined by /home/runner/work/time/time/BUILD
INFO: Elapsed time: 14.987s
INFO: 0 processes.
ERROR: Build did NOT complete successfully
ERROR: Build failed. Not running target