pyvm: split op handlers into ops_core/ops_box/ops_ctrl; add ops_flow + intrinsic; delegate vm.py without behavior change

net-plugin: modularize constants (consts.rs) and sockets (sockets.rs); remove legacy commented socket code; fix unused imports
mir: move instruction unit tests to tests/mir_instruction_unit.rs (file lean-up); no semantic changes
runner/pyvm: ensure using pre-strip; misc docs updates

Build: cargo build ok; legacy cfg warnings remain as before

apps/tests/macro/if/assign_join_basic.nyash

@@ -0,0 +1,5 @@
+function main(args) {
+  local a = 0
+  if 1 { a = 1 } else { a = 2 }
+}
+