feat(selfhost): field-move skip Drop, #line maps, Array field mono

Bring selfhost C backend closer to bootstrap ownership and debug quality.

- Mark locals moved into struct fields / returns (no double Drop)
- Mangle Array/Set/Map/Channel field types so user structs fully emit
- Emit #line N from HIR line; BUX_DEBUG_FILE / BUX_NO_LINE controls

src/hir_lower.bux

@@ -4073,7 +4073,18 @@ module HirLower {
                                 hm.structs[si].fields[fi].typeName = String_Concat(ftype.pointerPointee.typeName, "*");
                             }
                         } else if !String_Eq(ftype.typeName, "") {
-                            hm.structs[si].fields[fi].typeName = ftype.typeName;
+                            // Monomorphize container fields: Array<int> → Array_int
+                            // so C backend does not skip the parent struct as "generic".
+                            var tn: String = ftype.typeName;
+                            if ftype.typeArgCount >= 1 && !String_Eq(ftype.typeArgName0, "") {
+                                if String_Eq(tn, "Array") || String_Eq(tn, "Set") ||
+                                   String_Eq(tn, "Channel") || String_Eq(tn, "Iter") {
+                                    tn = String_Concat(String_Concat(tn, "_"), ftype.typeArgName0);
+                                } else if String_Eq(tn, "Map") && ftype.typeArgCount >= 2 {
+                                    tn = String_Concat(String_Concat(String_Concat("Map_", ftype.typeArgName0), "_"), ftype.typeArgName1);
+                                }
+                            }
+                            hm.structs[si].fields[fi].typeName = tn;
                         }
                     }
                     fi = fi + 1;