Add include directive for multi-file projects

Parser/preprocess module:
- New src/parser/preprocess.rs handles source-level include inlining
- 'include "path"' at line start splices the target file's content
- Paths resolved relative to the including file's directory
- Circular include detection via visited set (canonicalized paths)
- Non-cycle re-includes allowed (sibling branches can reuse files)

main.rs:
- Both compile() and check() run preprocess_source() before parsing
- Include resolution failures produce a clear error message

Tests: 4 new preprocess unit tests (249 total)
- passthrough, include line parsing, temp-file roundtrip

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/parser/mod.rs

@@ -1,7 +1,10 @@
 pub mod ast;
+pub mod preprocess;
 #[cfg(test)]
 mod tests;
 
+pub use preprocess::preprocess as preprocess_source;
+
 use crate::errors::{Diagnostic, ErrorCode};
 use crate::lexer::{Span, Token, TokenKind};
 use ast::*;