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/utils.rs

@@ -132,31 +132,31 @@ impl super::MirBuilder {
             if false && !self.suppress_pin_entry_copy_next {
                 // Keep old code for reference
                 // First pass: copy all pin slots and remember old->new mapping
-                let names: Vec<String> = self.variable_map.keys().cloned().collect();
+                let names: Vec<String> = self.variable_ctx.variable_map.keys().cloned().collect();
                 let mut pin_renames: Vec<(super::ValueId, super::ValueId)> = Vec::new();
                 for name in names.iter() {
                     if !name.starts_with("__pin$") {
                         continue;
                     }
-                    if let Some(&src) = self.variable_map.get(name) {
+                    if let Some(&src) = self.variable_ctx.variable_map.get(name) {
                         let dst = self.next_value_id();
                         self.emit_instruction(super::MirInstruction::Copy { dst, src })?;
                         crate::mir::builder::metadata::propagate::propagate(self, src, dst);
-                        self.variable_map.insert(name.clone(), dst);
+                        self.variable_ctx.variable_map.insert(name.clone(), dst);
                         pin_renames.push((src, dst));
                     }
                 }
                 // Second pass: update any user variables that pointed to old pin ids to the new ones
                 if !pin_renames.is_empty() {
                     let snapshot: Vec<(String, super::ValueId)> = self
-                        .variable_map
+                        .variable_ctx.variable_map
                         .iter()
                         .filter(|(k, _)| !k.starts_with("__pin$"))
                         .map(|(k, &v)| (k.clone(), v))
                         .collect();
                     for (k, v) in snapshot.into_iter() {
                         if let Some((_, newv)) = pin_renames.iter().find(|(oldv, _)| *oldv == v) {
-                            self.variable_map.insert(k, *newv);
+                            self.variable_ctx.variable_map.insert(k, *newv);
                         }
                     }
                 }
@@ -460,7 +460,7 @@ impl super::MirBuilder {
         // LocalSSA uses this to redirect old pinned values to the latest slot value.
         self.pin_slot_names.insert(v, slot_name.clone());
         self.pin_slot_names.insert(dst, slot_name.clone());
-        self.variable_map.insert(slot_name, dst);
+        self.variable_ctx.variable_map.insert(slot_name, dst);
         Ok(dst)
     }