test: Add host function tests for Rust-to-Zig interop

Add 10 tests demonstrating Rust host functions callable from Zig code
via `ZyntaxRuntime::with_symbols`. Tests cover:

- Basic host function calls (double, add, no params, 3 params)
- Host functions used in expressions
- Multiple host functions in same module
- Complex host functions (factorial)
- Host functions called in loops
- Mixed host and local functions
- Conditional host function dispatch

Uses `extern "C"` Rust functions registered via JIT symbol linking.

Total: 104 tests now passing for zyntax_embed

Author: darmie