chore: initialize NexaCore compiler workspace with basic frontend and CLI

Add initial project structure for NexaCore programming language compiler:
- Create Cargo workspace with 4 crates (cli, driver, frontend, runtime)
- Add lexer with indentation-based tokenization and keyword support
- Add parser for modules, functions, structs, and basic expressions
- Implement CLI with build command and placeholder subcommands
- Add driver crate to orchestrate compilation pipeline
- Include .gitignore for Rust build

crates/nxc-runtime/src/lib.rs

@@ -0,0 +1,10 @@
+//! NexaCore runtime scaffolding.
+//!
+//! TODO:
+//! - async scheduler
+//! - string and collection runtime
+//! - HTTP primitives
+//! - PostgreSQL client ABI
+
+pub const RUNTIME_VERSION: &str = "0.1.0";
+