feat: add parallel compilation with Rayon

[ryanorendorff]

Refactor preprocessor to compile code blocks in parallel using Rayon thread pool, achieving 2.46x speedup (1.24s -> 0.50s for 13 blocks in the test fixtures).

Module refactoring:

• Add compilation.rs: CompilationTask/Result types and parallel execution
• Add task_collector.rs: Task collection with validation limits
• Add reporting.rs: Error formatting and compilation statistics

Key improvements:

• Add parallel_jobs config option to control thread count
• Add CompilationTask::compile method for zero-copy field moves
• Use Arc for chapter paths to avoid cloning per task
• Collect all errors before failing to show all issues at once
• Add compilation statistics with per-language timing breakdown

Performance:

• Statistics show total time, average per block, and per-language breakdown
• Individual block timings available at debug log level