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 } => {