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/config/env.rs

@@ -173,6 +173,18 @@ pub fn mir_core13_pure() -> bool {
     std::env::var("NYASH_MIR_CORE13_PURE").ok().as_deref() == Some("1")
 }
 
+/// Enable heuristic pre-pin of comparison operands in if/loop headers.
+/// Default: OFF (0). Set NYASH_MIR_PREPIN=1 to enable.
+pub fn mir_pre_pin_compare_operands() -> bool {
+    match std::env::var("NYASH_MIR_PREPIN").ok() {
+        Some(v) => {
+            let lv = v.to_ascii_lowercase();
+            !(lv == "0" || lv == "false" || lv == "off")
+        }
+        None => false,
+    }
+}
+
 // ---- Optimizer diagnostics ----
 pub fn opt_debug() -> bool {
     std::env::var("NYASH_OPT_DEBUG").is_ok()