Use .is_empty() instead of .len() > 0 in assertions

## Context

In `src/lib.rs` (lines 231 and 403):

```rust
assert!(plan.render_plan.files.len() > 0);
assert!(result.files_created.len() > 0);
```

These should use `!files.is_empty()` for idiomatic Rust. Clippy likely isn't catching them because they're inside `assert!` macros.

## Suggested Fix

```rust
assert!(!plan.render_plan.files.is_empty());
assert!(!result.files_created.is_empty());
```