refactor(mir): Extract BindingContext from MirBuilder (Phase 136 follow-up 4/7)

## Summary
Extracted binding management into dedicated BindingContext struct,
completing step 4 of 7 in the Context Box refactoring plan.

## Changes
- NEW: src/mir/builder/binding_context.rs (BindingContext struct + helpers)
- Modified 7 files to use binding_ctx (SSOT pattern with legacy sync)
- Added comprehensive unit tests for BindingContext

## Extracted Fields
- binding_map: BTreeMap<String, BindingId> → binding_ctx.binding_map

## Benefits
- Clear separation: BindingId mapping isolated from MirBuilder
- Better testability: BindingContext can be tested independently
- Consistent pattern: Same SSOT + legacy sync approach as previous steps

## Tests
- cargo test --release --lib: 1008/1008 passed
- phase135_trim_mir_verify.sh: PASS
- Backward compatibility: 100% maintained (deprecated fields synced)

## Progress
Phase 136 Context Extraction: 4/7 complete (57%)
- ✅ Step 1: TypeContext
- ✅ Step 2: CoreContext
- ✅ Step 3: ScopeContext
- ✅ Step 4: BindingContext (this commit)
- ⏳ Step 5: VariableContext
- ⏳ Step 6: MetadataContext
- ⏳ Step 7: RegionContext

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

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

src/mir/builder/control_flow/joinir/patterns/pattern2_with_break.rs

@@ -115,7 +115,8 @@ fn prepare_pattern2_inputs(
 
     // Phase 79-2: Register loop variable BindingId (dev-only)
     #[cfg(feature = "normalized_dev")]
-    if let Some(loop_var_bid) = builder.binding_map.get(&loop_var_name).copied() {
+    // Phase 136 Step 4/7: Use binding_ctx for lookup
+    if let Some(loop_var_bid) = builder.binding_ctx.lookup(&loop_var_name) {
         env.register_loop_var_binding(loop_var_bid, _loop_var_join_id);
         log_pattern2(
             verbose,

src/mir/builder/control_flow/joinir/patterns/pattern3_with_if_phi.rs

@@ -133,8 +133,9 @@ impl MirBuilder {
         #[cfg(feature = "normalized_dev")]
         {
             // Register loop variable BindingId
-            if let Some(bid) = self.binding_map.get(&loop_var_name) {
-                cond_env.register_loop_var_binding(*bid, _loop_var_join_id);
+            // Phase 136 Step 4/7: Use binding_ctx for lookup
+            if let Some(bid) = self.binding_ctx.lookup(&loop_var_name) {
+                cond_env.register_loop_var_binding(bid, _loop_var_join_id);
                 if debug {
                     eprintln!(
                         "[phase80/p3] Registered loop var '{}' BindingId({}) -> ValueId({})",
@@ -147,8 +148,9 @@ impl MirBuilder {
             // These are variables from the condition expression (e.g., "len" in "i < len")
             // May include ConditionOnly carriers if they appear in the condition
             for binding in &condition_bindings {
-                if let Some(bid) = self.binding_map.get(&binding.name) {
-                    cond_env.register_condition_binding(*bid, binding.join_value);
+                // Phase 136 Step 4/7: Use binding_ctx for lookup
+                if let Some(bid) = self.binding_ctx.lookup(&binding.name) {
+                    cond_env.register_condition_binding(bid, binding.join_value);
                     if debug {
                         eprintln!(
                             "[phase80/p3] Registered condition binding '{}' BindingId({}) -> ValueId({})",

src/mir/builder/control_flow/joinir/patterns/pattern4_with_continue.rs

@@ -240,7 +240,8 @@ fn prepare_pattern4_context(
             continue_cond,
             loop_body: &normalized_body,
             #[cfg(feature = "normalized_dev")]
-            binding_map: Some(&builder.binding_map),
+            // Phase 136 Step 4/7: Use binding_ctx for binding_map reference
+            binding_map: Some(builder.binding_ctx.binding_map()),
         };
 
         match LoopBodyCondPromoter::try_promote_for_condition(promotion_req) {
@@ -339,7 +340,8 @@ fn lower_pattern4_joinir(
     let mut join_value_space = JoinValueSpace::new();
 
     #[cfg(feature = "normalized_dev")]
-    let binding_map_clone = builder.binding_map.clone();
+    // Phase 136 Step 4/7: Use binding_ctx for binding_map clone
+    let binding_map_clone = builder.binding_ctx.binding_map().clone();
 
     let (join_module, exit_meta) = match lower_loop_with_continue_minimal(
         prepared.loop_scope.clone(),

src/mir/builder/control_flow/joinir/patterns/trim_loop_lowering.rs

@@ -227,7 +227,8 @@ impl TrimLoopLowerer {
             break_cond: Some(break_cond),
             loop_body: body,
             #[cfg(feature = "normalized_dev")]
-            binding_map: Some(&builder.binding_map),
+            // Phase 136 Step 4/7: Use binding_ctx for binding_map reference
+            binding_map: Some(builder.binding_ctx.binding_map()),
         };
 
         match LoopBodyCarrierPromoter::try_promote(&request) {
@@ -239,7 +240,8 @@ impl TrimLoopLowerer {
 
                 // Step 3: Convert to CarrierInfo and merge
                 #[cfg(feature = "normalized_dev")]
-                let promoted_carrier = trim_info.to_carrier_info(Some(&builder.binding_map));
+                // Phase 136 Step 4/7: Use binding_ctx for binding_map reference
+                let promoted_carrier = trim_info.to_carrier_info(Some(builder.binding_ctx.binding_map()));
                 #[cfg(not(feature = "normalized_dev"))]
                 let promoted_carrier = trim_info.to_carrier_info();
                 carrier_info.merge_from(&promoted_carrier);