builder/vm: stabilize json_lint_vm under unified calls

- Fix condition_fn resolution: Value call path + dev safety + stub injection
- VM bridge: handle Method::birth via BoxCall; ArrayBox push/get/length/set direct bridge
- Receiver safety: pin receiver in method_call_handlers to avoid undefined use across blocks
- Local vars: materialize on declaration (use init ValueId; void for uninit)
- Prefer legacy BoxCall for Array/Map/String/user boxes in emit_box_or_plugin_call (stability-first)
- Test runner: update LLVM hint to llvmlite harness (remove LLVM_SYS_180_PREFIX guidance)
- Docs/roadmap: update CURRENT_TASK with unified default-ON + guards

Note: NYASH_DEV_BIRTH_INJECT_BUILTINS=1 can re-enable builtin birth() injection during migration.

src/tests/refcell_assignment_test.rs

@@ -1,3 +1,5 @@
+#![cfg(feature = "interpreter-legacy")]
+
 use crate::interpreter::NyashInterpreter;
 use crate::ast::{ASTNode, LiteralValue};
 use crate::box_trait::{NyashBox, IntegerBox};