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/libs/string_builder.nyash

@@ -0,0 +1,15 @@
+// StringBuilderBox: simple buffered concatenation
+static box StringBuilderBox {
+  birth() { me._buf = [] }
+  append(s) { me._buf.push(s)  return me }
+  toString() {
+    // join by empty string
+    local n = me._buf.size()
+    if n == 0 { return "" }
+    local out = me._buf.get(0)
+    local i = 1
+    loop (i < n) { out = out + me._buf.get(i)  i = i + 1 }
+    return out
+  }
+}
+