feat: Add zyntax_embed SDK with language grammar interface

- Add zyntax_embed crate for embedding Zyntax as a JIT runtime
- Implement LanguageGrammar API for parsing with ZynPEG grammars
- Add ZyntaxRuntime and TieredRuntime for JIT compilation
- Add ZyntaxValue, ZyntaxString, ZyntaxArray for type interop
- Add ZyntaxPromise for async operations with .then()/.catch()
- Add FromZyntax/IntoZyntax traits for bidirectional conversion
- Extend lowering module with AstLowering trait and helpers
- Add compile_with_grammar for full source-to-native pipeline
- Add comprehensive book chapter (12-embedding-sdk.md)
- Add architecture documentation (EMBEDDING_SDK.md)
- Include External Function registration examples

Author: darmie

Cargo.toml

@@ -1,6 +1,6 @@
 [workspace]
 resolver = "2"
-members = ["crates/typed_ast", "crates/compiler", "crates/whirlwind_adapter", "crates/zyntax_cli", "crates/runtime", "crates/zyntax_plugin_macros", "crates/zyn_peg", "crates/haxe_zyntax_runtime"]
+members = ["crates/typed_ast", "crates/compiler", "crates/whirlwind_adapter", "crates/zyntax_cli", "crates/runtime", "crates/zyntax_plugin_macros", "crates/zyn_peg", "crates/haxe_zyntax_runtime", "crates/zyntax_embed", "sdk/zrtl", "sdk/zrtl_macros"]
 
 
 [workspace.dependencies]