Integrate horologium

[rhannequin]

Time scale conversions that hold by definition or by model belong in horologium rather than in Astronoby, so TAI and TDB are now read from it. UT1 stays it: it follows the actual rotation of the Earth and rests on observed ΔT, which is not what horologium is for.

Instant keeps its Terrestrial Time Julian Date as before and builds the horologium instant on the first reading that needs it.
The paths that only want TT never touch it, so the rise/transit/set, twilight, moon phase, eclipse and position workloads stay where they were.

tdb used to return Terrestrial Time unchanged, documented in the source as an approximation. It now returns real TDB from the Fairhead and Bretagnon series ERFA uses. Precession is evaluated in TT, correction.

horologium requires iers 0.2, so the minimum iers version is raised from 0.1 to 0.2.

[Copilot]

🟡 Changes recommended

Standard precision breaks the documented exactness guarantee for TAI conversions.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Integrates horologium for TAI/TDB conversion while retaining Astronoby’s TT and UT1 handling.

Changes:

• Adds accurate TDB conversion and Rational outputs.
• Updates precession to use TT.
• Raises iers to 0.2 and updates documentation.

File summaries

File	Description
UPGRADING.md	Documents migration and precision changes.
spec/astronoby/instant_spec.rb	Tests TDB conversion and output type.
lib/astronoby/precession.rb	Evaluates precession using TT.
lib/astronoby/instant.rb	Delegates TAI/TDB conversions to horologium.
lib/astronoby/constants.rb	Removes the local TAI–TT offset.
Gemfile.lock	Locks new dependency versions.
docs/instant.md	Updates time-scale documentation.
docs/glossary.md	Adds TAI and TDB definitions.
CHANGELOG.md	Records user-visible changes.
astronoby.gemspec	Adds horologium and raises iers.

Review details

• Files reviewed: 9/10 changed files
• Comments generated: 2
• Review effort level: Balanced

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