feat(clock): add shared clock primitives and clock-aware timestamps

[schmidma]

Description

This change adds a small, explicit time layer to ros-z: ZClock, ZTime, ZDuration, and context/node plumbing so all entities created from a context can share the same clock source.

ros-z now provides a consistent clock abstraction and uses it for attachment timestamps, but it does not introduce an executor, replay system, or higher-level runtime model. That keeps the API generally useful while making simulated or externally controlled time possible for downstream runtimes.

Scope is intentionally narrow. This PR only adds the time substrate and the minimum wiring needed to propagate it through publishers, services, parameter services, and type description services. It avoids adding capture, replay, or other helpers.

Checklist

• Ran ./scripts/check-local.sh successfully
• Added/updated tests/documentation (if applicable)

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 89 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/ros-z/src/time.rs	64.89%	66 Missing ⚠️
crates/ros-z/src/context.rs	46.15%	7 Missing ⚠️
crates/ros-z/src/attachment.rs	61.53%	5 Missing ⚠️
crates/ros-z/src/node.rs	77.77%	4 Missing ⚠️
crates/ros-z/src/service.rs	66.66%	4 Missing ⚠️
...ates/ros-z/src/dynamic/type_description_service.rs	0.00%	2 Missing ⚠️
...rates/ros-z/src/dynamic/type_description_client.rs	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
crates/ros-z/src/parameter/service.rs	35.03% <100.00%> (+1.07%)	⬆️
crates/ros-z/src/pubsub.rs	45.68% <100.00%> (+1.20%)	⬆️
...rates/ros-z/src/dynamic/type_description_client.rs	22.74% <0.00%> (-0.08%)	⬇️
...ates/ros-z/src/dynamic/type_description_service.rs	30.67% <0.00%> (-0.12%)	⬇️
crates/ros-z/src/node.rs	44.18% <77.77%> (+0.49%)	⬆️
crates/ros-z/src/service.rs	78.64% <66.66%> (-0.43%)	⬇️
crates/ros-z/src/attachment.rs	83.78% <61.53%> (-16.22%)	⬇️
crates/ros-z/src/context.rs	49.18% <46.15%> (-0.18%)	⬇️
crates/ros-z/src/time.rs	64.89% <64.89%> (ø)

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[YuanYuYuan]

Hi @schmidma, could you please address the test coverage? Thanks