fix(mir): fix else block scope bug - PHI materialization order

Root Cause:
- Else blocks were not propagating variable assignments to outer scope
- Bug 1 (if_form.rs): PHI materialization happened before variable_map reset,
  causing PHI nodes to be lost
- Bug 2 (phi.rs): Variable merge didn't check if else branch modified variables

Changes:
- src/mir/builder/if_form.rs:93-127
  - Reordered: reset variable_map BEFORE materializing PHI nodes
  - Now matches then-branch pattern (reset → materialize → execute)
  - Applied to both "else" and "no else" branches for consistency
- src/mir/builder/phi.rs:137-154
  - Added else_modified_var check to detect variable modifications
  - Use modified value from else_var_map_end_opt when available
  - Fall back to pre-if value only when truly not modified

Test Results:
✅ Simple block: { x=42 } → 42
✅ If block: if 1 { x=42 } → 42
✅ Else block: if 0 { x=99 } else { x=42 } → 42 (FIXED!)
✅ Stage-B body extraction: "return 42" correctly extracted (was null)

Impact:
- Else block variable assignments now work correctly
- Stage-B compiler body extraction restored
- Selfhost builder path can now function
- Foundation for Phase 21.x progress

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/builder/phi.rs

@@ -137,13 +137,20 @@ impl MirBuilder {
                 .get(&var_name)
                 .copied()
                 .unwrap_or(then_value_raw);
+            // Check if else branch actually modified the variable (even if not as last expression)
+            let else_modified_var = else_var_map_end_opt
+                .as_ref()
+                .and_then(|m| m.get(&var_name).copied());
             let else_value_for_var = if else_assigns_same {
                 else_var_map_end_opt
                     .as_ref()
                     .and_then(|m| m.get(&var_name).copied())
                     .unwrap_or(else_value_raw)
+            } else if let Some(else_modified) = else_modified_var {
+                // Else modifies the variable (even if not as the last expression)
+                else_modified
             } else {
-                // Else doesn't assign: use pre-if value if available
+                // Else doesn't modify the variable: use pre-if value if available
                 pre_then_var_value.unwrap_or(else_value_raw)
             };
             // Build inputs from reachable predecessors only