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/ssa/local.rs

@@ -54,16 +54,16 @@ pub fn ensure(builder: &mut MirBuilder, v: ValueId, kind: LocalKind) -> ValueId
         }
 
         // CRITICAL FIX: If `v` is from a pinned slot, check if there's a PHI value for that slot
-        // in the current block's variable_map. If so, use the PHI value directly instead of
+        // in the current block's variable_ctx.variable_map. If so, use the PHI value directly instead of
         // emitting a Copy from the old value (which might not be defined in this block).
         // Try to detect pinned slots for this value and redirect to the latest slot value.
-        // 1) First, look for "__pin$" entries in variable_map that still point to v.
+        // 1) First, look for "__pin$" entries in variable_ctx.variable_map that still point to v.
         // 2) If not found, consult builder.pin_slot_names to recover the slot name
         //    and then look up the current ValueId for that slot.
         let mut slot_name_opt: Option<String> = None;
 
         let names_for_v: Vec<String> = builder
-            .variable_map
+            .variable_ctx.variable_map
             .iter()
             .filter(|(k, &vid)| vid == v && k.starts_with("__pin$"))
             .map(|(k, _)| k.clone())
@@ -76,7 +76,7 @@ pub fn ensure(builder: &mut MirBuilder, v: ValueId, kind: LocalKind) -> ValueId
         }
 
         if let Some(slot_name) = slot_name_opt {
-            if let Some(&current_val) = builder.variable_map.get(&slot_name) {
+            if let Some(&current_val) = builder.variable_ctx.variable_map.get(&slot_name) {
                 if current_val != v {
                     // The slot has been updated (likely by a PHI or header rewrite).
                     // Use the updated value instead of the stale pinned ValueId.