Prompt Template Engine — Variable Injection

## Task
Build a prompt template engine for LLM applications.

## Requirements
- Language: TypeScript
- Zero dependencies (or minimal: just a parser)
- Type-safe variable injection

## Template Syntax
```
Simple variables: {{name}}, {{company}}
Conditionals: {{#if premium}}...{{else}}...{{/if}}
Loops: {{#each items}}{{this.name}}: {{this.value}}{{/each}}
Includes: {{> partials/header}}
Defaults: {{name | default: "User"}}
Transforms: {{name | uppercase}}, {{text | truncate: 100}}
Comments: {{! This is ignored }}
```

## API
```typescript
const engine = new PromptEngine();

// Register template
engine.register("review-prompt", `
You are reviewing {{product}} for {{#if expert}}an expert{{else}}a general{{/if}} audience.
{{#each criteria}}
- Evaluate: {{this}}
{{/each}}
`);

// Render
const prompt = engine.render("review-prompt", {
  product: "iPhone 16",
  expert: true,
  criteria: ["design", "performance", "value"],
});

// Validate — check all required variables are provided
engine.validate("review-prompt", variables); // throws if missing
```

## Implementation Notes
1. Parse template into AST (tokens: text, variable, block-open, block-close)
2. Compile AST into a render function for performance
3. Type-safe: infer variable types from template
4. Escape HTML/XML in variables by default (opt-out with {{{raw}}})
5. Max nesting depth of 5 to prevent infinite loops
6. Include a CLI tool: `prompt-render template.txt data.json`