feat(cse): pass all 3 reproducer tests (pure, branching, reference)

All CSE tests pass with correct structure preservation:
- cse-simple-callee.rs: double(x), 1 rule, baseline=reuse structure
- cse-branching-callee.rs: classify(x) with if/else, 2 rules from
  2 cover paths, reuse produces correct splits
- cse-reference-args.rs: add_to(&x, y) with reference argument,
  slotStore-based reference handling works correctly

Fix: sanitize module/rule names for Kore identifier requirements

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Stevengre

kmir/src/kmir/_cse.py

@@ -356,8 +356,15 @@ def _build_summary_rule(
 
 
 def _sanitize_name(name: str) -> str:
-    """Sanitize function name for use as K rule label."""
-    return name.replace('::', '-').replace('<', '').replace('>', '').replace(' ', '')
+    """Sanitize function name for use as K identifiers (module names, rule labels)."""
+    import re
+
+    result = name.replace('::', '-').replace('<', '').replace('>', '').replace(' ', '').replace('_', '-')
+    result = re.sub(r'[^a-zA-Z0-9-]', '', result)
+    # Kore identifiers must start with a letter
+    if result and not result[0].isalpha():
+        result = 'cse' + result
+    return result
 
 
 def build_summary_module(callee_name: str, rules: list[KRule]) -> KFlatModule:

kmir/src/tests/integration/data/prove-rs/cse-branching-callee.rs

@@ -0,0 +1,13 @@
+fn classify(x: u32) -> u32 {
+    if x > 10 {
+        1
+    } else {
+        0
+    }
+}
+
+fn main() {
+    let a: u32 = 5;
+    let result = classify(a);
+    assert!(result == 0);
+}

kmir/src/tests/integration/data/prove-rs/cse-reference-args.rs

@@ -0,0 +1,10 @@
+fn add_to(x: &u32, y: u32) -> u32 {
+    *x + y
+}
+
+fn main() {
+    let a: u32 = 3;
+    let b: u32 = 7;
+    let result = add_to(&a, b);
+    assert!(result == 10);
+}