feat(plan): Phase 273 P2 Step 0 - CoreEffectPlan side effect support

Prepare CoreEffectPlan::MethodCall for Pattern7 (split) support:
- dst: ValueId → Option<ValueId> (for void methods like push)
- effects: EffectMask field added (PURE+Io vs MUT)

Changes:
- plan/mod.rs: MethodCall struct updated
- plan/lowerer.rs: emit_effect() uses dst/effects from plan
- plan/normalizer.rs: MethodCall with explicit effects
- plan/verifier.rs: Handle Option<ValueId> dst

Test: phase258_p0_index_of_string_vm PASS (exit=6)

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

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

src/mir/builder/control_flow/plan/lowerer.rs

@@ -14,7 +14,7 @@
 use super::{CoreEffectPlan, CoreLoopPlan, CorePlan};
 use crate::mir::builder::control_flow::joinir::patterns::router::LoopPatternContext;
 use crate::mir::builder::MirBuilder;
-use crate::mir::{Effect, EffectMask, MirInstruction, ValueId};
+use crate::mir::{MirInstruction, ValueId};
 
 /// Phase 273 P1: PlanLowerer - CorePlan → MIR 生成 (SSOT)
 pub(in crate::mir::builder) struct PlanLowerer;
@@ -218,14 +218,15 @@ impl PlanLowerer {
                     value: value.clone(),
                 })?;
             }
-            CoreEffectPlan::MethodCall { dst, object, method, args } => {
+            CoreEffectPlan::MethodCall { dst, object, method, args, effects } => {
+                // P2: dst and effects are now specified by Normalizer
                 builder.emit_instruction(MirInstruction::BoxCall {
-                    dst: Some(*dst),
+                    dst: *dst,
                     box_val: *object,
                     method: method.clone(),
                     method_id: None,
                     args: args.clone(),
-                    effects: EffectMask::PURE.add(Effect::Io),
+                    effects: *effects,
                 })?;
             }
             CoreEffectPlan::BinOp { dst, lhs, op, rhs } => {

src/mir/builder/control_flow/plan/mod.rs

@@ -24,7 +24,7 @@
 //! This prevents "second language processor" from growing inside Lowerer.
 
 use crate::ast::ASTNode;
-use crate::mir::{BasicBlockId, BinaryOp, CompareOp, ConstValue, ValueId};
+use crate::mir::{BasicBlockId, BinaryOp, CompareOp, ConstValue, EffectMask, ValueId};
 
 pub(in crate::mir::builder) mod lowerer;
 pub(in crate::mir::builder) mod normalizer;
@@ -186,14 +186,21 @@ pub(in crate::mir::builder) struct CoreIfPlan {
 ///
 /// Effect vocabulary is minimal (scan-specific variants forbidden):
 /// - MethodCall, BinOp, Compare, Const only
+///
+/// Phase 273 P2: MethodCall now supports:
+/// - dst: Option<ValueId> for void methods (e.g., push)
+/// - effects: EffectMask for side effects (e.g., MUT for push)
 #[derive(Debug, Clone)]
 pub(in crate::mir::builder) enum CoreEffectPlan {
     /// Method call (args are ValueIds, not Strings!)
+    ///
+    /// Phase 273 P2: dst is Option for void methods, effects for side effects
     MethodCall {
-        dst: ValueId,
+        dst: Option<ValueId>,   // P2: Option for void methods (push)
         object: ValueId,
-        method: String, // Method name only (OK as String)
+        method: String,         // Method name only (OK as String)
         args: Vec<ValueId>,
+        effects: EffectMask,    // P2: Side effect mask (PURE+Io or MUT)
     },
 
     /// Binary operation

src/mir/builder/control_flow/plan/normalizer.rs

@@ -16,7 +16,7 @@ use super::{
 };
 use crate::mir::builder::control_flow::joinir::patterns::router::LoopPatternContext;
 use crate::mir::builder::MirBuilder;
-use crate::mir::{BinaryOp, CompareOp, ConstValue, MirType};
+use crate::mir::{BinaryOp, CompareOp, ConstValue, Effect, EffectMask, MirType};
 
 /// Phase 273 P1: PlanNormalizer - DomainPlan → CorePlan 変換 (SSOT)
 pub(in crate::mir::builder) struct PlanNormalizer;
@@ -189,19 +189,21 @@ impl PlanNormalizer {
         // needle_len = needle.length() if dynamic, else reuse one_val
         if parts.dynamic_needle {
             header_effects.push(CoreEffectPlan::MethodCall {
-                dst: needle_len_val,
+                dst: Some(needle_len_val),
                 object: needle_host,
                 method: "length".to_string(),
                 args: vec![],
+                effects: EffectMask::PURE.add(Effect::Io),
             });
         }
 
         // len = s.length()
         header_effects.push(CoreEffectPlan::MethodCall {
-            dst: len_val,
+            dst: Some(len_val),
             object: s_host,
             method: "length".to_string(),
             args: vec![],
+            effects: EffectMask::PURE.add(Effect::Io),
         });
 
         // bound = len - needle_len
@@ -231,10 +233,11 @@ impl PlanNormalizer {
             }),
             // window = s.substring(i, i + needle_len)
             CorePlan::Effect(CoreEffectPlan::MethodCall {
-                dst: window_val,
+                dst: Some(window_val),
                 object: s_host,
                 method: "substring".to_string(),
                 args: vec![i_current, i_plus_needle_len],
+                effects: EffectMask::PURE.add(Effect::Io),
             }),
             // cond_match = window == needle
             CorePlan::Effect(CoreEffectPlan::Compare {

src/mir/builder/control_flow/plan/verifier.rs

@@ -142,8 +142,11 @@ impl PlanVerifier {
     /// V6: Effect ValueId validity
     fn verify_effect(effect: &CoreEffectPlan, depth: usize) -> Result<(), String> {
         match effect {
-            CoreEffectPlan::MethodCall { dst, object, method, args } => {
-                Self::verify_value_id_basic(*dst, depth, "MethodCall.dst")?;
+            CoreEffectPlan::MethodCall { dst, object, method, args, effects: _ } => {
+                // P2: dst is now Option<ValueId>
+                if let Some(dst_val) = dst {
+                    Self::verify_value_id_basic(*dst_val, depth, "MethodCall.dst")?;
+                }
                 Self::verify_value_id_basic(*object, depth, "MethodCall.object")?;
                 if method.is_empty() {
                     return Err(format!(