mir/vm: SSA pin+PHI + short-circuit; user-defined method calls → functions; entry single-pred PHIs; compare-operand pin; VM BoxCall fallback to InstanceBox methods; docs: update CURRENT_TASK (plan + acceptance)

- Lower And/Or to branch+PHI (RHS not evaluated)
- Always slotify compare operands (dominance safety)
- Insert single-predecessor PHIs at then/else/short-circuit entries
- pin_to_slot now logs (NYASH_PIN_TRACE) and participates in PHI
- Rewrite user-defined instance method calls to Box.method/Arity (builder)
- VM fallback: BoxCall on InstanceBox dispatches to lowered functions with 'me'+args
- Keep plugin/BoxCall path for core boxes (String/Array/Map)
- Add env-gated pre-pin for if/loop (NYASH_MIR_PREPIN)
- CURRENT_TASK: add SSA/userbox plan, debug steps, acceptance criteria

src/mir/builder/phi.rs

@@ -25,6 +25,8 @@ impl MirBuilder {
             then_map_end,
             else_map_end_opt,
         );
+        use std::collections::HashSet;
+        let changed_set: HashSet<String> = changed.iter().cloned().collect();
         for name in changed {
             if skip_var.map(|s| s == name).unwrap_or(false) {
                 continue;
@@ -50,6 +52,29 @@ impl MirBuilder {
             self.emit_instruction(MirInstruction::Phi { dst: merged, inputs })?;
             self.variable_map.insert(name, merged);
         }
+
+        // Ensure pinned synthetic slots ("__pin$...") have a block-local definition at the merge,
+        // even if their values did not change across branches. This avoids undefined uses when
+        // subsequent blocks re-use pinned values without modifications.
+        for (pin_name, pre_val) in pre_if_snapshot.iter() {
+            if !pin_name.starts_with("__pin$") { continue; }
+            if skip_var.map(|s| s == pin_name.as_str()).unwrap_or(false) { continue; }
+            if changed_set.contains(pin_name) { continue; }
+            let then_v = then_map_end.get(pin_name.as_str()).copied().unwrap_or(*pre_val);
+            let else_v = else_map_end_opt
+                .as_ref()
+                .and_then(|m| m.get(pin_name.as_str()).copied())
+                .unwrap_or(*pre_val);
+            let then_pred = then_exit_block;
+            let else_pred = else_exit_block_opt.unwrap_or(else_block);
+            let merged = self.value_gen.next();
+            let inputs = vec![(then_pred, then_v), (else_pred, else_v)];
+            if let (Some(func), Some(cur_bb)) = (&self.current_function, self.current_block) {
+                crate::mir::phi_core::common::debug_verify_phi_inputs(func, cur_bb, &inputs);
+            }
+            self.emit_instruction(MirInstruction::Phi { dst: merged, inputs })?;
+            self.variable_map.insert(pin_name.clone(), merged);
+        }
         Ok(())
     }
     /// Normalize Phi creation for if/else constructs.