Introduce Center, Body and Position domain objects

[rhannequin]

Summary

This PR improves three existing concepts in model, turning incidental behaviour into a guaranteed, documented contract:

• Astronoby::Center: a value object replacing the ad-hoc center_identifier union on reference frames.
• Astronoby::Body: a role for a catalog definition that can position itself at an instant.
• Astronoby::Position: a role for a target resolved at an instant, yielding the reference frame chain.

Motivation

Two concepts leaked through the otherwise clean value-object design.

First, the centre of a frame was a primitive union. Frames carried center_identifier, an Integer (SolarSystemBody::EARTH / SOLAR_SYSTEM_BARYCENTER) for barycentric/geocentric frames or an [longitude, latitude] array for topocentric frames.

Second, "an observable target" was implicit. SolarSystemBody and DeepSkyObjectPosition both produced the frame chain but shared no abstraction, so cross-type operations worked only by coincidence.

Changes

Center value object

• New Astronoby::Center with barycentric / geocentric (memoized singletons) and topocentric(observer) constructors, plus #barycentric?, #geocentric?, #topocentric?, #observer_dependent? and #observer.
• Two topocentric centres are equal when their observers share the same latitude, longitude and elevation. Atmospheric parameters (temperature, pressure) are intentionally ignored, since they do not affect the geometric centre.
• ReferenceFrame#separation_from now compares frames with center == other.center.
• All frame builders (Geometric, Astrometric, MeanOfDate, Apparent, Topocentric, Teme) and DeepSkyObjectPosition updated accordingly.

Body role

• New marker module Astronoby::Body for a definition that responds to at(instant, ephem:) and returns a Position.
• SolarSystemBody extends it (the body classes themselves are Bodys); DeepSkyObject includes it (instances are Bodys).

Position role

• New module Astronoby::Position, included by SolarSystemBody and DeepSkyObjectPosition, holding the shared observed_by implementation (previously duplicated in both classes).

target_body made consistent

• ReferenceFrame#target_body is now always a Body: solar system body frames carry the class, deep-sky object frames carry the DeepSkyObject definition (threaded into DeepSkyObjectPosition).
• LightTimeDelay is unchanged. It reads target_body.geometric(ephem:, instant:) only off Geometric frames, which already carried the class. What used to be an implicit reliance is now a documented role contract.

Breaking changes

• ReferenceFrame#center_identifier is replaced by #center, which returns an Astronoby::Center instead of an Integer/Array. See UPGRADING.md.
• ReferenceFrame#target_body is now consistently an Astronoby::Body (previously a class or an instance depending on the frame).

# Before
frame.center_identifier # => 399 or [longitude, latitude]

# After
frame.center                                  # => #<Astronoby::Center ...>
frame.center.geocentric?                      # => true
frame.center == Astronoby::Center.geocentric  # => true
frame.center.observer                         # => the Observer, for topocentric frames

[Copilot]

Pull request overview

This PR introduces three new domain abstractions—Astronoby::Center, Astronoby::Body, and Astronoby::Position—to formalize previously incidental contracts around reference-frame centers and “targets resolved at an instant”. It refactors the reference frame chain to use Center instead of the center_identifier union, and standardizes how frames expose their target_body across solar-system and deep-sky targets.

Changes:

• Replace ReferenceFrame#center_identifier with #center returning a new Astronoby::Center value object, and update all frame builders accordingly.
• Introduce Astronoby::Body (marker role) and Astronoby::Position (shared observed_by behavior), wiring them into solar-system bodies and deep-sky objects/positions.
• Update specs and documentation (UPGRADING.md, reference frame docs, glossary) to reflect the new contracts.

Reviewed changes

Copilot reviewed 24 out of 24 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
UPGRADING.md	Documents the breaking change from center_identifier to center and Astronoby::Center semantics.
docs/reference_frames.md	Adds #center to the documented reference-frame interface.
docs/glossary.md	Defines the new “Body”, “Centre/Center”, and “Position” concepts for user-facing docs.
lib/astronoby.rb	Requires the new center, body, and position definitions.
lib/astronoby/center.rb	Adds the Astronoby::Center value object (barycentric/geocentric/topocentric).
lib/astronoby/body.rb	Adds the Astronoby::Body marker module.
lib/astronoby/position.rb	Adds the Astronoby::Position role with shared observed_by implementation.
lib/astronoby/reference_frame.rb	Swaps center_identifier for center and compares centers via value semantics in separation_from.
lib/astronoby/reference_frames/topocentric.rb	Threads Center.topocentric(observer) through the Topocentric frame construction and initializer.
lib/astronoby/reference_frames/teme.rb	Migrates TEME frames to use Center (geocentric/topocentric) instead of center_identifier.
lib/astronoby/reference_frames/mean_of_date.rb	Migrates MeanOfDate builder to use Center.geocentric.
lib/astronoby/reference_frames/geometric.rb	Migrates Geometric frame to use Center.barycentric.
lib/astronoby/reference_frames/astrometric.rb	Migrates Astrometric builder to use Center.geocentric.
lib/astronoby/reference_frames/apparent.rb	Migrates Apparent builder to use Center.geocentric.
lib/astronoby/bodies/solar_system_body.rb	Makes solar-system body instances Position, body classes Body, and threads target_body as the body definition.
lib/astronoby/bodies/deep_sky_object.rb	Makes DeepSkyObject instances fulfill Body and threads the definition into DeepSkyObjectPosition.
lib/astronoby/bodies/deep_sky_object_position.rb	Makes deep-sky positions fulfill Position and changes frames to carry the body definition as target_body.
spec/astronoby/reference_frames/teme_spec.rb	Updates expectations to assert center == Astronoby::Center.geocentric.
spec/astronoby/reference_frames/mean_of_date_spec.rb	Updates argument expectations from center_identifier: to center:.
spec/astronoby/reference_frames/astrometric_spec.rb	Updates argument expectations from center_identifier: to center:.
spec/astronoby/reference_frame_spec.rb	Adds a comparison test asserting differing-elevation observers yield incompatible centers.
spec/astronoby/position_spec.rb	Adds tests for the Position role and target_body being a Body throughout the chain.
spec/astronoby/center_spec.rb	Adds tests for Center constructors, equality, and hashing semantics.
spec/astronoby/body_spec.rb	Adds tests asserting solar-system body classes and deep-sky objects fulfill Body.

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[Copilot]

Pull request overview

Copilot reviewed 24 out of 24 changed files in this pull request and generated 2 comments.