feat: add lexer implementation

PLAN.md

@@ -8,10 +8,10 @@ A Rust-flavored, C-targeting language - built pipeline-first.
 ## Phase 1 - Lexer
 
 - [x] Define token enum (int literal, bool literal, ident, keywords, operators, punctuation)
-- [ ] Implement character-by-character scanner loop
-- [ ] Handle whitespace & single-line comments (`//`)
-- [ ] Produce source spans (file, line, col) on every token
-- [ ] Unit-test: known inputs → expected token streams
+- [x] Implement character-by-character scanner loop
+- [x] Handle whitespace & single-line comments (`//`)
+- [x] Produce source spans (file, line, col) on every token
+- [x] Unit-test: known inputs → expected token streams
 
 ## Phase 2 - Parser