refactor(cleanup): Remove unused BindingContext methods

Removed dead code from BindingContext after analysis of actual usage:

**Deleted Methods (3)**:
- contains() - never used
- len() - never used
- is_empty() - never used

**Feature Gate Fix**:
- binding_map(): Added #[cfg(feature = "normalized_dev")]
  - Method only used within normalized_dev feature gates
  - Prevents "never used" warning in standard builds

**Analysis**:
BindingContext is actively used via:
- lookup() - variable name to BindingId resolution
- insert() - variable registration
- remove() - variable removal
- binding_map() - internal map access (normalized_dev only)

Files using binding_ctx:
- vars/lexical_scope.rs (5 usages)
- pattern3_with_if_phi.rs (2 usages)
- pattern4_with_continue.rs (2 usages)
- trim_loop_lowering.rs (2 usages)
- pattern2_inputs_facts_box.rs (1 usage)

**Result**: All BindingContext dead_code warnings eliminated

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/mir/builder/binding_context.rs

@@ -73,24 +73,12 @@ impl BindingContext {
     }
 
     /// Get immutable reference to the binding map (for BindingMapProvider)
-    pub fn binding_map(&self) -> &BTreeMap<String, BindingId> {
+    /// Phase 136: Used only with normalized_dev feature
+    #[cfg(feature = "normalized_dev")]
+    pub(super) fn binding_map(&self) -> &BTreeMap<String, BindingId> {
         &self.binding_map
     }
 
-    /// Check if a variable has a binding
-    pub fn contains(&self, name: &str) -> bool {
-        self.binding_map.contains_key(name)
-    }
-
-    /// Get the number of bindings
-    pub fn len(&self) -> usize {
-        self.binding_map.len()
-    }
-
-    /// Check if there are no bindings
-    pub fn is_empty(&self) -> bool {
-        self.binding_map.is_empty()
-    }
 }
 
 #[cfg(test)]