M2: Wire IR pipeline, add Coin Cavern example and integration tests

Pipeline:
- main.rs now runs IR lowering and optimization before codegen
- IR is built and optimized but output still uses AST-based codegen
  (IR-based codegen is a future improvement)

Coin Cavern example (examples/coin_cavern.ne):
- 3-state game: Title → Playing → GameOver
- Functions (clamp_x), constants, gravity physics, coin collection
- Demonstrates most M2 language features

Integration tests (14 total, 7 new):
- program_with_functions: functions with params and return values
- program_with_while_loop: while loops compile correctly
- program_with_fast_slow_vars: placement hints accepted
- program_with_multi_state_transitions: 3-state cycle
- coin_cavern_compiles: full Coin Cavern example
- ir_pipeline_produces_ir: validates IR lowering + optimizer
- error_test_recursion_detected: E0402 for recursive functions

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/main.rs

@@ -4,7 +4,9 @@ use std::path::PathBuf;
 use nescript::analyzer;
 use nescript::codegen::CodeGen;
 use nescript::errors::render_diagnostics;
+use nescript::ir;
 use nescript::linker::Linker;
+use nescript::optimizer;
 
 #[derive(Parser)]
 #[command(name = "nescript", about = "NEScript compiler — NES game development")]
@@ -87,7 +89,11 @@ fn compile(input: &PathBuf) -> Result<Vec<u8>, ()> {
         return Err(());
     }
 
-    // Code generation
+    // IR lowering and optimization
+    let mut ir_program = ir::lower(&program, &analysis);
+    optimizer::optimize(&mut ir_program);
+
+    // Code generation (still AST-based for M2; IR codegen comes in M3)
     let codegen = CodeGen::new(&analysis.var_allocations, &program.constants);
     let instructions = codegen.generate(&program);