feat: Next-Gen Polyglot XML Schema Compiler & Toolchain (Epic #22)

[nth-bailey]

Summary

Closes #22
Closes #23
Closes #24
Closes #25
Closes #26
Closes #27
Closes #28
Closes #29
Closes #30
Closes #31
Closes #32

This PR delivers the Next-Gen Polyglot XML Schema Compiler & Toolchain for PolyXML, introducing a high-performance W3C XSD 1.0 & 1.1 compiler and code generator in pure Rust. It parses XML Schemas into a canonical, language-agnostic Intermediate Representation (IR), resolves recursive cycles via Tarjan's Strongly Connected Components algorithm, and generates idiomatic data models and high-performance codecs across 7 modern programming languages:

🎯 Supported Language Targets

1. Python 3.12+ ([Target: Python] Modern Python 3.12+ Codegen (@dataclass & Pydantic v2) #24):

   • Modern @dataclass (stdlib) and Pydantic v2 models.
   • Comprehensive XSD facet validation (pattern regex, minInclusive, maxInclusive, minLength, maxLength).
   • Zero-copy streaming deserializer and serializer codecs ([Phase 3 / Runtime Codecs] Zero-Copy Streaming Deserializer & Serializer Codec Synthesis for Rust & Python #32).
   • Formatted via Ruff.

2. Rust 2021/2024 ([Target: Rust] Zero-Copy Rust 2021/2024 Codegen (struct + enum with Cow<'a, str>) #25, [Phase 3 / Runtime Codecs] Zero-Copy Streaming Deserializer & Serializer Codec Synthesis for Rust & Python #32):

   • Zero-copy Cow<'a, str> and owned struct modes.
   • Automatic Tarjan SCC recursive cycle boxing (Box<T>).
   • Zero-copy streaming deserializer (FromXmlStream) and serializer (ToXmlStream) codecs ([Phase 3 / Runtime Codecs] Zero-Copy Streaming Deserializer & Serializer Codec Synthesis for Rust & Python #32).
   • Formatted via cargo fmt with 0 clippy warnings.

3. Modern C++20 / C++23 ([Target: C++] Modern C++20/C++23 Codegen (struct, std::variant, std::optional, Concepts) #26):

   • Value types, std::variant choice representations, and C++20 concepts (PolyXmlElement).
   • Smart pointer cycle cuts (std::unique_ptr<T>).
   • CMake and Meson project export integration.
   • Formatted via clang-format.

4. Java 21+ ([Target: Java] Java 21+ Modern Codegen (record + sealed interface) #27):

   • Modern immutable Java records and sealed interface choice models with exhaustive pattern matching.
   • Cyclic reference graph resolution.
   • Jakarta Bean Validation annotations (@Pattern, @Size, @Min, @Max).
   • Formatted via Google Java Format.

5. TypeScript 5+ ([Target: TypeScript] TypeScript 5+ & Zod Codegen (Discriminated Unions & Strict Types) #28):

   • Idiomatic TypeScript interfaces and discriminated unions.
   • Production Zod runtime validation schemas with z.lazy() recursion.
   • Formatted via Prettier.

6. Go 1.22+ ([Target: Go] Go 1.22+ Struct Codegen (struct with XML tags & Choice Validation) #29):

   • Idiomatic Go structs with standard encoding/xml tags and pointer cycle cuts.
   • Custom UnmarshalXML choice types guaranteeing mutual exclusivity.
   • Formatted via gofmt.

7. C# 12 / .NET 8+ ([Target: C#] C# 12 / .NET 8+ Codegen (record class / record struct) #30):

   • Modern record definitions with primary constructors and System.Xml.Serialization attributes.
   • Polymorphic xs:choice abstract records.
   • IValidatableObject implementation with regex and facet constraints.
   • Formatted via csharpier.

---

🛠️ CLI & Workspace Toolchain (#31)

• polyxml generate: Multi-language, single-invocation code generation with --lang, --out, --backend, --zero-copy, --codecs, and --dry-run.
• polyxml build: Declarative workspace orchestration powered by polyxml.toml.
• polyxml validate: Fast static XML Schema structure and cycle topology validation.

---

🛡️ Quality Gate & Verification

• Rust: cargo fmt --check, cargo clippy --workspace --all-targets -D warnings (0 warnings).
• Python: ruff check, ruff format --check, and 100% statement & branch coverage (pytest --cov --cov-branch --cov-fail-under=100).
• Workspace Tests: All unit and integration tests passing, including end-to-end compilation tests for Python, Rust, C++, Java, TypeScript, Go, and C#.