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/control_flow/joinir/patterns/pattern1_minimal.rs

@@ -54,7 +54,7 @@ impl MirBuilder {
         let ctx = build_pattern_context(self, condition, body, PatternVariant::Pattern1)?;
 
         // Phase 195: Use unified trace
-        trace::trace().varmap("pattern1_start", &self.variable_map);
+        trace::trace().varmap("pattern1_start", &self.variable_ctx.variable_map);
 
         // Phase 202-A: Create JoinValueSpace for unified ValueId allocation
         // Pattern 1 uses Param region for boundary input slots (loop var) and Local region for temps.

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

@@ -108,7 +108,7 @@ fn prepare_pattern2_inputs(
         ConditionEnvBuilder::build_for_break_condition_v2(
             condition,
             &loop_var_name,
-            &builder.variable_map,
+            &builder.variable_ctx.variable_map,
             loop_var_id,
             &mut join_value_space,
         )?;
@@ -140,7 +140,7 @@ fn prepare_pattern2_inputs(
 
     // Add captured vars
     for var in &captured_env.vars {
-        if let Some(&host_id) = builder.variable_map.get(&var.name) {
+        if let Some(&host_id) = builder.variable_ctx.variable_map.get(&var.name) {
             let join_id = join_value_space.alloc_param();
             env.insert(var.name.clone(), join_id);
             condition_bindings.push(ConditionBinding {
@@ -676,7 +676,7 @@ pub(crate) fn can_lower(builder: &MirBuilder, ctx: &super::router::LoopPatternCo
     CommonPatternInitializer::check_carrier_updates_allowed(
         ctx.body,
         &loop_var_name,
-        &builder.variable_map,
+        &builder.variable_ctx.variable_map,
     )
 }
 
@@ -745,7 +745,7 @@ impl MirBuilder {
         use super::pattern_pipeline::{build_pattern_context, PatternVariant};
         let ctx = build_pattern_context(self, condition, _body, PatternVariant::Pattern2)?;
 
-        trace::trace().varmap("pattern2_start", &self.variable_map);
+        trace::trace().varmap("pattern2_start", &self.variable_ctx.variable_map);
 
         let mut inputs =
             prepare_pattern2_inputs(self, condition, _body, fn_body, &ctx, verbose)?;
@@ -977,11 +977,11 @@ mod tests {
         use crate::mir::ValueId;
 
         let mut builder = MirBuilder::new();
-        builder.variable_map.insert("i".to_string(), ValueId(1));
-        builder.variable_map.insert("len".to_string(), ValueId(2));
-        builder.variable_map.insert("s".to_string(), ValueId(3));
-        builder.variable_map.insert("digits".to_string(), ValueId(4));
-        builder.variable_map.insert("result".to_string(), ValueId(5));
+        builder.variable_ctx.variable_map.insert("i".to_string(), ValueId(1));
+        builder.variable_ctx.variable_map.insert("len".to_string(), ValueId(2));
+        builder.variable_ctx.variable_map.insert("s".to_string(), ValueId(3));
+        builder.variable_ctx.variable_map.insert("digits".to_string(), ValueId(4));
+        builder.variable_ctx.variable_map.insert("result".to_string(), ValueId(5));
 
         let condition = bin(BinaryOperator::Less, var("i"), var("len"));
 

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

@@ -124,7 +124,7 @@ impl MirBuilder {
             ConditionEnvBuilder::build_for_break_condition_v2(
                 condition,
                 &loop_var_name,
-                &self.variable_map,
+                &self.variable_ctx.variable_map,
                 loop_var_id,
                 &mut join_value_space,
             )?;
@@ -183,7 +183,7 @@ impl MirBuilder {
             // Collect parent-defined variables from function scope
             // For now, use all variables in variable_map except loop_var
             let parent_defined: Vec<String> = self
-                .variable_map
+                .variable_ctx.variable_map
                 .keys()
                 .filter(|name| *name != &loop_var_name)
                 .cloned()

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

@@ -81,7 +81,7 @@ pub(crate) fn can_lower(builder: &MirBuilder, ctx: &super::router::LoopPatternCo
     CommonPatternInitializer::check_carrier_updates_allowed(
         ctx.body,
         &loop_var_name,
-        &builder.variable_map,
+        &builder.variable_ctx.variable_map,
     )
 }
 
@@ -335,7 +335,7 @@ fn lower_pattern4_joinir(
     use crate::mir::join_ir::lowering::loop_with_continue_minimal::lower_loop_with_continue_minimal;
     use crate::mir::join_ir::lowering::JoinInlineBoundaryBuilder;
 
-    trace::trace().varmap("pattern4_start", &builder.variable_map);
+    trace::trace().varmap("pattern4_start", &builder.variable_ctx.variable_map);
 
     let mut join_value_space = JoinValueSpace::new();
 

src/mir/builder/control_flow/joinir/patterns/pattern5_infinite_early_exit.rs

@@ -316,9 +316,9 @@ pub(crate) fn lower(
         );
     }
 
-    // Step 2: Get counter ValueId from variable_map
-    let counter_id = builder.variable_map.get(&counter_name).copied().ok_or_else(|| {
-        format!("Counter variable '{}' not found in variable_map", counter_name)
+    // Step 2: Get counter ValueId from variable_ctx.variable_map
+    let counter_id = builder.variable_ctx.variable_map.get(&counter_name).copied().ok_or_else(|| {
+        format!("Counter variable '{}' not found in variable_ctx.variable_map", counter_name)
     })?;
 
     if debug {

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

@@ -263,7 +263,7 @@ pub(crate) fn build_pattern_context(
     let (loop_var_name, loop_var_id, carrier_info) = CommonPatternInitializer::initialize_pattern(
         builder,
         condition,
-        &builder.variable_map,
+        &builder.variable_ctx.variable_map,
         None, // No exclusions for now (Pattern 2/4 will filter carriers later)
     )?;
 

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

@@ -280,7 +280,7 @@ impl TrimLoopLowerer {
                 Self::generate_carrier_initialization(builder, body, trim_helper)?;
 
                 eprintln!(
-                    "[TrimLoopLowerer] Registered carrier '{}' in variable_map",
+                    "[TrimLoopLowerer] Registered carrier '{}' in variable_ctx.variable_map",
                     trim_helper.carrier_name
                 );
 
@@ -328,7 +328,7 @@ impl TrimLoopLowerer {
     /// Generates:
     /// 1. ch0 = s.substring(start, start+1)
     /// 2. is_ch_match0 = (ch0 == " " || ch0 == "\t" || ...)
-    /// 3. Registers carrier in variable_map
+    /// 3. Registers carrier in variable_ctx.variable_map
     fn generate_carrier_initialization(
         builder: &mut MirBuilder,
         body: &[ASTNode],
@@ -353,7 +353,7 @@ impl TrimLoopLowerer {
 
         // Get ValueIds for string and start
         let s_id =
-            builder.variable_map.get(&s_name).copied().ok_or_else(|| {
+            builder.variable_ctx.variable_map.get(&s_name).copied().ok_or_else(|| {
                 format!("[TrimLoopLowerer] String variable '{}' not found", s_name)
             })?;
 
@@ -401,9 +401,9 @@ impl TrimLoopLowerer {
             is_ch_match0
         );
 
-        // Register carrier in variable_map
+        // Register carrier in variable_ctx.variable_map
         builder
-            .variable_map
+            .variable_ctx.variable_map
             .insert(trim_helper.carrier_name.clone(), is_ch_match0);
 
         Ok(())
@@ -438,7 +438,7 @@ impl TrimLoopLowerer {
         let mut bindings = Vec::new();
 
         // Add carrier to ConditionEnv
-        let get_value = |name: &str| builder.variable_map.get(name).copied();
+        let get_value = |name: &str| builder.variable_ctx.variable_map.get(name).copied();
         let mut env_temp = std::collections::HashMap::new(); // Temporary env for closure
 
         let binding = TrimPatternLowerer::add_to_condition_env(