Expand RS 3001 corpus to all base-relevant examples; refresh README #43

Context

The RS 3001 conformance corpus currently covers 18 fixtures (§Comment, §Class, §Attribute forms, §Value, §Enums, §Instance, §Package, require/models). lutaml-lang.adoc has roughly 45 base-relevant normative examples; the remainder (§Serialization basics that don't need mapping blocks, remaining §Attribute argument forms, §Cardinality, §Reference forms) are not vendored, so regressions there are not caught by CI.

Also: the README still describes the pre-PARG architecture (Parslet-era wording, no skip-trivia/conformance story).

Scope

• vendor remaining examples as spec/fixtures/rs3001/*.lml (the corpus spec auto-discovers them); extension-track examples stay excluded by convention
• README: current architecture (PARG + skip trivia + compiled artifact), conformance + validation story, quick start