Add fuzz testing and parser edge case regression tests

Fuzz targets (cargo +nightly fuzz):
- fuzz_lexer: lexer must never panic on arbitrary input
- fuzz_parser: parser must never panic on arbitrary input
- fuzz_compile: full pipeline (parse → analyze → IR → optimize →
  codegen → assemble → link) must never panic

Results: 3.3M iterations across all targets, zero crashes.
Seed corpus from all 7 examples + crafted edge cases.

11 new parser edge case tests:
- draw_followed_by_statement: regression for keyword-arg lookahead bug
- draw_with_frame_followed_by_statement: same with frame: arg
- nested_if_else_chain: 3-deep else-if chain
- deeply_nested_blocks: 4-deep nested if blocks
- empty_function_body: fun noop() {}
- empty_state_handlers: on enter/exit/frame {}
- button_start_in_condition: "start" keyword as button name
- button_select_in_condition: "select" keyword as button name
- multiple_draws_in_sequence: 3 draws followed by assignment
- parser_no_panic_on_garbage: 25 malformed inputs
- lexer_no_panic_on_garbage: 10 malformed inputs

221 tests total, all pre-commit checks pass.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

fuzz/fuzz_targets/fuzz_parser.rs

@@ -0,0 +1,10 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|data: &[u8]| {
+    // The parser must never panic — always produce an AST or diagnostics.
+    if let Ok(source) = std::str::from_utf8(data) {
+        let _ = nescript::parser::parse(source);
+    }
+});