Added model generation from rnc schema

[HassanAkbar]

fixes #55

[ronaldtse]

@HassanAkbar are you planning to merge this into #699 (I know the base targets it) first, then mark #699 as ready to merge? Thanks!

[HassanAkbar]

@ronaldtse yup, that's the plan.
But if we need #699 is needed, I can finalize it first and then merge this in main. What do you suggest here?

[Copilot]

Pull request overview

Adds RELAX NG Compact Syntax (RNC) schema import by adapting the existing RNG compiler pipeline (RNC → Rng::Grammar → RngCompiler), including preprocessing for RFC XML–style RNC features and an entry point via Lutaml::Model::Schema.from_rnc.

Changes:

• Introduces Lutaml::Model::Schema::RncCompiler with preprocessing (annotations stripping, occurrence normalization, include expansion) and wiring via Schema.from_rnc.
• Extends RNG attribute handling to preserve xml:-prefixed attributes in generated mappings.
• Adds documentation and comprehensive RSpec coverage + fixtures for RNC compilation scenarios (includes, warnings, load/create_files modes).

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
spec/lutaml/xml/schema/rnc_compiler_spec.rb	Adds end-to-end and unit-level tests for RNC compilation, preprocessing behaviors, include resolution, and Schema.from_rnc.
spec/fixtures/xml/schema/rnc/address_book.rnc	Fixture RNC schema for baseline model generation.
spec/fixtures/xml/schema/rnc/book_features.rnc	Fixture exercising RFC XML-style RNC features (annotations, mixed attribute choices, escaped identifiers, xml:lang).
spec/fixtures/xml/schema/rnc/includes/library.rnc	Fixture for include resolution.
spec/fixtures/xml/schema/rnc/includes/book.rnc	Included fixture referenced by library.rnc.
spec/fixtures/xml/schema/rnc/includes/circular_a.rnc	Fixture for circular include detection.
spec/fixtures/xml/schema/rnc/includes/circular_b.rnc	Fixture for circular include detection.
lib/lutaml/xml.rb	Registers Schema.from_rnc in the XML plugin layer (with Opal guard).
lib/lutaml/model/schema.rb	Adds autoload + placeholder from_rnc when lutaml-xml isn’t loaded.
lib/lutaml/model/schema/rng_compiler.rb	Allows alternate default output dir via :default_output_dir (used by RNC adapter).
lib/lutaml/model/schema/rng_compiler/element_visitor.rb	Preserves xml: prefix in generated attribute mappings (e.g., xml:lang).
lib/lutaml/model/schema/rnc_compiler.rb	New RNC compiler adapter delegating to RngCompiler, with start-define cleanup and warnings plumbing.
lib/lutaml/model/schema/rnc_compiler/preprocessor.rb	New preprocessing layer for RNC constructs the rng parser doesn’t handle well.
lib/lutaml/model/schema/rnc_compiler/source_resolver.rb	New helper for resolving base directories / sources for include expansion.
docs/_guides/schema-import.adoc	Documents RNG/RNC import and the location: option behavior.

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