Read and write an instant as ISO 8601

[rhannequin]

Add the ISO 8601 representation. instant.as(:iso8601, scale:) writes an instant as an extended ISO 8601 date and time, and Instant.from_iso8601 reads one back. It formats the calendar date the civil representation reads, so their fields match.

The scale is not part of the string. There is no ISO 8601 designator for TAI or TT, and Z means UTC, so a bare time is a coordinate in the scale passed to the call. This matches astropy.time, which keeps the scale on the object rather than in the string. UTC will write Z when it lands.

The fraction of a second is written to nanosecond resolution. The instant is rounded onto the nanosecond grid first, so a fraction that would round up moves into the next second. The parser reads a strict subset: a full date, an optional time after a T, a fraction of any length, and an optional Z or numeric offset applied as arithmetic. Anything else raises ParseError.

[Copilot]

Pull request overview

Adds an ISO 8601 string representation for Horologium::Instant, enabling round-trippable serialization via instant.as(:iso8601, scale:) and parsing via Instant.from_iso8601, aligned with the library’s “scale lives on the object, not the string” model.

Changes:

• Introduces Representations::Iso8601 to render instants as extended ISO 8601 with nanosecond resolution and parse a strict subset (date, optional time, optional fraction, optional offset).
• Adds Instant.from_iso8601 and registers :iso8601 as a ScaleReading representation.
• Adds comprehensive ISO 8601 tests and updates README and RBS signatures accordingly.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/test_scale_reading.rb	Updates known representations list to include :iso8601.
test/representations/test_iso8601.rb	Adds coverage for ISO 8601 rendering/parsing behavior and refusal cases.
sig/horologium/scale_reading.rbs	Extends REPRESENTATIONS type union to include Iso8601.
sig/horologium/representations/iso8601.rbs	Adds RBS for the new ISO 8601 representation API.
sig/horologium/instant.rbs	Adds Instant.from_iso8601 signature.
README.md	Documents ISO 8601 usage and the supported parsing subset.
lib/horologium/scale_reading.rb	Registers the new :iso8601 representation.
lib/horologium/representations/iso8601.rb	Implements ISO 8601 rendering/parsing logic.
lib/horologium/instant.rb	Adds Instant.from_iso8601 entry point.
lib/horologium/data/barycentric_model.rb	Minor documentation wording change.
lib/horologium.rb	Requires the new representation file.

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