VM: PHI strict default-ON + loop-header PHI fix; add VM step budget; StringBox.lastIndexOf; docs + strict smoke

- PHI strict: default ON, disable via HAKO_VM_PHI_STRICT=0 (alias NYASH_VM_PHI_STRICT=0)
- LoopForm: insert initial header PHI (preheader input) and rebind vars before condition; seal updates PHI inputs; avoid duplicate PHIs by replace
- MIR interpreter: add HAKO_VM_MAX_STEPS (alias NYASH_VM_MAX_STEPS) fail-fast budget to prevent infinite loops
- StringBox.lastIndexOf implemented (rfind, returns -1 when not found) in VM handlers
- Smokes: add strict/core/vm_phi_strict_smoke.sh (opt-in); quick remains green 120/120
- Docs: lang/src/vm/README.md and CURRENT_TASK.md updated with PHI strict policy and step budget

src/backend/mir_interpreter/helpers.rs

@@ -37,7 +37,9 @@ impl MirInterpreter {
                     );
                 }
                 // Dev-time safety valve: tolerate undefined registers as Void when enabled
-                let tolerate = std::env::var("NYASH_VM_TOLERATE_VOID").ok().as_deref() == Some("1");
+                let tolerate = std::env::var("NYASH_VM_TOLERATE_VOID").ok().as_deref() == Some("1")
+                    || std::env::var("HAKO_PHI_VERIFY").ok().map(|v| v.to_ascii_lowercase()).map(|v| v=="1"||v=="true"||v=="on").unwrap_or(false)
+                    || std::env::var("NYASH_PHI_VERIFY").ok().map(|v| v.to_ascii_lowercase()).map(|v| v=="1"||v=="true"||v=="on").unwrap_or(false);
                 if tolerate {
                     return Ok(VMValue::Void);
                 }