refactor: decouple core from PyO3, introduce ryx-rs native API, and expand type system

[Einswilli]

Here is a professional and structured Pull Request title and description in English, summarizing all the architectural ## Overview
This PR represents a major architectural milestone for Ryx. We have successfully decoupled the core ORM logic from the Python bindings (pyo3), paving the way for a fully native Rust ORM experience while preserving the performance and correctness of the Python API.

Additionally, we've introduced ryx-rs (the native Rust Django-style API) and ryx-macro for clean model declarations, expanded our SQL type-casting system, and updated our documentation to reflect these changes.

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Key Changes

1. Architectural Decoupling & Modularization

• Created ryx-common: Houses shared types like RyxError, PoolConfig, ModelMeta, and FieldMeta. This removes any direct dependencies on pyo3 or ryx-core from our fundamental layers.
• Created ryx-rs: A pure, native Rust implementation of the Django-style ORM API.
• Created ryx-macro: Contains macro definitions such as #[model] and #[relation] to dramatically simplify model declaration in Rust.
• Refactored ryx-core: Stripped down to act purely as a PyO3 bridge between Rust and Python.
• Python-Optional Backends: Modified ryx-backend to make ryx-core optional via a python feature flag, making the backend completely usable in pure Rust environments.

2. Expanded SQL Type System & Support

• New SqlValue Variants: Expanded SqlValue to support specialized types: Date, DateTime, Time, Uuid, Decimal, and Json.
• Database Casting: Updated Postgres, MySQL, and SQLite drivers to support proper binding, type casting, and heuristic decoding for these new variants.
• Python Integration: Added custom error mapping (err_to_py / IntoPyResult) and sql_to_py conversions in ryx-python to properly handle these types across the PyO3 boundary.

3. Advanced Query Features in Rust (select_related)

• Expanded the FromRow trait to support nested decoding via from_row_joined and from_row_prefixed.
• Updated QueryNode and the SQL compiler to handle column-aliasing via extra_aliases.
• Implemented select_related in ryx-rs's QuerySet, enabling single-query LEFT JOINs that automatically hydrate related nested structs.

4. Tests, Benchmarks, & CI

• Added comprehensive relation tests in ryx-rs/tests/relation_test.rs.
• Cleaned up database migration tests.
• Added performance benchmarks in ryx-backend/benches/.
• Corrected test paths in .github/workflows/ci.yml and updated the release workflow.

5. Documentation Overhaul

• Landing Page: Streamlined and simplified the main README.md to highlight both Python and Rust quick-start examples, key features, and visual architecture.
• Rust API Reference: Added complete Rust-centric documentation pages (docs/doc/rust/) detailing installation, CRUD, filtering, relationships, caching, and migrations.
• Updated sidebars and existing Python guides to match the latest API changes.

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How to Test

1. Verify Python tests:

   uv run pytest ryx-python/tests/ -v

2. Verify Rust tests:

   cargo test --workspace

3. Run benchmarks:

   cargo bench -p ryx-backend