refactor(mir): Remove VariableContext legacy fields (Phase 2-6/7)

完全移行→削除の安全順序（Option C）に従い、VariableContext の
deprecated フィールドと sync helpers を完全削除。

## Changes
- Migrated all 66+ access sites to variable_ctx.variable_map
- Removed 1 deprecated field (variable_map) from MirBuilder
- Removed 2 sync helpers (sync_variable_ctx_to_legacy, sync_legacy_to_variable_ctx)
- Fixed BoxCompilationContext.variable_map references (kept as-is, different scope)
- Fixed ExitBindingBuilder.variable_map references (kept as-is, local field)
- Updated observer.rs to use variable_map() accessor method

## JoinIR Integration Verified
- CarrierInfo::from_variable_map() works correctly
- ExitLine contract maintained (Phase 132-135)
- NYASH_TRACE_VARMAP debug support preserved
- Pattern 1-5 lowering all functional

## Tests
- cargo test --release --lib: 1033 passed, 0 failed
- phase135_trim_mir_verify.sh: PASS (MIR SSA/ValueId OK)
- cargo build --release: SUCCESS
- Deprecation warnings: reduced (86 remaining, from other contexts)

## Statistics
- 27 files changed
- 146 insertions(+), 174 deletions(-)
- Net: -28 lines

Phase 2 Progress: 6/7 contexts complete (86%)
- ✅ MetadataContext
- ✅ CoreContext
- ✅ TypeContext
- ✅ ScopeContext
- ✅ BindingContext
- ✅ VariableContext (this commit)
- ⏳ CompilationContext (Phase 2-7 next)

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

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

src/mir/builder/phi.rs

@@ -64,7 +64,7 @@ impl MirBuilder {
                 0 => {}
                 1 => {
                     let (_pred, v) = inputs[0];
-                    self.variable_map.insert(pin_name.clone(), v);
+                    self.variable_ctx.variable_map.insert(pin_name.clone(), v);
                 }
                 _ => {
                     if let Some(func) = self.scope_ctx.current_function.as_mut() {
@@ -94,7 +94,7 @@ impl MirBuilder {
                             type_hint: None, // Phase 63-6: Legacy path, no type hint
                         })?;
                     }
-                    self.variable_map.insert(pin_name.clone(), merged);
+                    self.variable_ctx.variable_map.insert(pin_name.clone(), merged);
                 }
             }
         }
@@ -164,8 +164,8 @@ impl MirBuilder {
                 0 => {}
                 1 => {
                     // Direct bind (no PHI needed)
-                    self.variable_map = pre_if_var_map.clone();
-                    self.variable_map.insert(var_name, inputs[0].1);
+                    self.variable_ctx.variable_map = pre_if_var_map.clone();
+                    self.variable_ctx.variable_map.insert(var_name, inputs[0].1);
                     return Ok(inputs[0].1);
                 }
                 _ => {
@@ -181,8 +181,8 @@ impl MirBuilder {
                     self.insert_phi_with_dst(result_val, inputs)?;
                 }
             }
-            self.variable_map = pre_if_var_map.clone();
-            self.variable_map.insert(var_name, result_val);
+            self.variable_ctx.variable_map = pre_if_var_map.clone();
+            self.variable_ctx.variable_map.insert(var_name, result_val);
         } else {
             // No variable assignment pattern detected – just emit Phi for expression result
             let mut inputs: Vec<(BasicBlockId, ValueId)> = Vec::new();
@@ -215,7 +215,7 @@ impl MirBuilder {
                 }
             }
             // Merge variable map conservatively to pre-if snapshot (no new bindings)
-            self.variable_map = pre_if_var_map.clone();
+            self.variable_ctx.variable_map = pre_if_var_map.clone();
         }
 
         Ok(result_val)