M2: Add call graph analysis, recursion detection, and optimizer

Analyzer extensions:
- Call graph construction from function bodies and state handlers
- DFS-based recursion detection (direct and mutual) with E0402 errors
- Max call depth computation per entry point with E0401 enforcement
- Function declarations registered as symbols (E0503 for undefined calls)
- Collects calls from all statement/expression types recursively

Optimizer (new module):
- Constant folding: evaluate known-constant arithmetic at compile time
- Dead code elimination: remove ops with unused destination temps
- Both operate per-basic-block in a single pass

171 tests total (22 new: 6 analyzer + 11 IR lowering + 5 optimizer)

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/ir/mod.rs

@@ -133,7 +133,7 @@ pub enum IrOp {
 pub enum IrTerminator {
     /// Unconditional jump to a label.
     Jump(String),
-    /// Conditional branch: if temp != 0 goto true_label else goto false_label.
+    /// Conditional branch: if temp != 0 goto `true_label` else goto `false_label`.
     Branch(IrTemp, String, String),
     /// Return from function, optionally with a value.
     Return(Option<IrTemp>),