feat(stdlib): generic Iter_Map/Filter/Fold with func monomorphization

Add Iter_Map<T,U>, Filter, Fold, Any, All, ForEach over fat function
pointers; keep Iter_MapInt and friends as thin aliases.

Compiler fixes required for non-int returns and capturing closures:
- bootstrap: resolve fat-func call return type under mono typeSubst
- bootstrap: type-check args of Foo<T>(...) so closures capture correctly
- selfhost: substitute type params inside tekFunc (BuxFn_U_T → concrete)
- selfhost: emit cstr fat typedefs with #ifndef redefinition guards

Example: examples/iter_generic.bux (int↔String map, fold, closures).
Selfhost-loop remains binary-identical.

src/c_backend.bux

@@ -774,32 +774,17 @@ func CBE_FatPartToC(part: String) -> String {
 // Emit typedefs for common BuxFn_* shapes (fat function pointers)
 func CBE_EmitFatFuncTypedefs(cbe: *CEmitter, mod: *HirModule) {
     StringBuilder_Append(&cbe.sb, "/* Fat function pointer types (code + env) */\n");
-    // (int)->int
-    StringBuilder_Append(&cbe.sb, "typedef struct BuxFn_int_int {\n");
-    StringBuilder_Append(&cbe.sb, "    int (*code)(void* env, int a0);\n");
-    StringBuilder_Append(&cbe.sb, "    void* env;\n");
-    StringBuilder_Append(&cbe.sb, "} BuxFn_int_int;\n");
-    // (int,int)->int
-    StringBuilder_Append(&cbe.sb, "typedef struct BuxFn_int_int_int {\n");
-    StringBuilder_Append(&cbe.sb, "    int (*code)(void* env, int a0, int a1);\n");
-    StringBuilder_Append(&cbe.sb, "    void* env;\n");
-    StringBuilder_Append(&cbe.sb, "} BuxFn_int_int_int;\n");
-    // (int)->bool
-    StringBuilder_Append(&cbe.sb, "typedef struct BuxFn_bool_int {\n");
-    StringBuilder_Append(&cbe.sb, "    bool (*code)(void* env, int a0);\n");
-    StringBuilder_Append(&cbe.sb, "    void* env;\n");
-    StringBuilder_Append(&cbe.sb, "} BuxFn_bool_int;\n");
-    // ()->void
-    StringBuilder_Append(&cbe.sb, "typedef struct BuxFn_void_void {\n");
-    StringBuilder_Append(&cbe.sb, "    void (*code)(void* env);\n");
-    StringBuilder_Append(&cbe.sb, "    void* env;\n");
-    StringBuilder_Append(&cbe.sb, "} BuxFn_void_void;\n");
-    // ()->int
-    StringBuilder_Append(&cbe.sb, "typedef struct BuxFn_int_void {\n");
-    StringBuilder_Append(&cbe.sb, "    int (*code)(void* env);\n");
-    StringBuilder_Append(&cbe.sb, "    void* env;\n");
-    StringBuilder_Append(&cbe.sb, "} BuxFn_int_void;\n");
-    // Scan module for any other BuxFn_* names
+    // Always emit core shapes
+    CBE_EmitOneFatTypedef(cbe, "BuxFn_int_int");
+    CBE_EmitOneFatTypedef(cbe, "BuxFn_int_int_int");
+    CBE_EmitOneFatTypedef(cbe, "BuxFn_bool_int");
+    CBE_EmitOneFatTypedef(cbe, "BuxFn_void_void");
+    CBE_EmitOneFatTypedef(cbe, "BuxFn_int_void");
+    CBE_EmitOneFatTypedef(cbe, "BuxFn_cstr_int");
+    CBE_EmitOneFatTypedef(cbe, "BuxFn_int_cstr");
+    CBE_EmitOneFatTypedef(cbe, "BuxFn_bool_cstr");
+    CBE_EmitOneFatTypedef(cbe, "BuxFn_int_int_cstr");
+    // Scan module for any other BuxFn_* names (deduped via #ifndef in EmitOne)
     var i: int = 0;
     while i < mod.funcCount {
         CBE_MaybeEmitExtraFat(cbe, mod.funcs[i].retTypeName);
@@ -826,12 +811,6 @@ func CBE_EmitFatFuncTypedefs(cbe: *CEmitter, mod: *HirModule) {
 func CBE_MaybeEmitExtraFat(cbe: *CEmitter, name: String) {
     if String_Eq(name, "") { return; }
     if !String_StartsWith(name, "BuxFn_") { return; }
-    // Skip ones we already emit as built-ins
-    if String_Eq(name, "BuxFn_int_int") { return; }
-    if String_Eq(name, "BuxFn_int_int_int") { return; }
-    if String_Eq(name, "BuxFn_bool_int") { return; }
-    if String_Eq(name, "BuxFn_void_void") { return; }
-    if String_Eq(name, "BuxFn_int_void") { return; }
     CBE_EmitOneFatTypedef(cbe, name);
 }
 
@@ -859,6 +838,13 @@ func CBE_EmitOneFatTypedef(cbe: *CEmitter, fatName: String) {
     let retPart: String = String_SplitPart(rest, "_", 0);
     let retC: String = CBE_FatPartToC(retPart);
 
+    // Guard against redefinition if the same name is emitted twice
+    StringBuilder_Append(&cbe.sb, "#ifndef ");
+    StringBuilder_Append(&cbe.sb, fatName);
+    StringBuilder_Append(&cbe.sb, "_DEFINED\n#define ");
+    StringBuilder_Append(&cbe.sb, fatName);
+    StringBuilder_Append(&cbe.sb, "_DEFINED\n");
+
     StringBuilder_Append(&cbe.sb, "typedef struct ");
     StringBuilder_Append(&cbe.sb, fatName);
     StringBuilder_Append(&cbe.sb, " {\n    ");
@@ -876,7 +862,7 @@ func CBE_EmitOneFatTypedef(cbe: *CEmitter, fatName: String) {
     }
     StringBuilder_Append(&cbe.sb, ");\n    void* env;\n} ");
     StringBuilder_Append(&cbe.sb, fatName);
-    StringBuilder_Append(&cbe.sb, ";\n");
+    StringBuilder_Append(&cbe.sb, ";\n#endif\n");
 }
 
 func CBE_EmitMakerDecl(cbe: *CEmitter, f: *HirFunc) {

src/hir_lower.bux

@@ -152,6 +152,35 @@ func Lcx_SubstituteType(ctx: *LowerCtx, te: *TypeExpr) -> *TypeExpr {
         return r;
     }
 
+    // Fat function type: func(T)->U — substitute params and return
+    if te.kind == tekFunc {
+        let r: *TypeExpr = bux_alloc(sizeof(TypeExpr)) as *TypeExpr;
+        r.kind = tekFunc;
+        r.line = te.line;
+        r.column = te.column;
+        r.funcParamCount = te.funcParamCount;
+        r.funcRet = Lcx_SubstituteType(ctx, te.funcRet);
+        var head: *TypeExprList = null as *TypeExprList;
+        var tail: *TypeExprList = null as *TypeExprList;
+        var cur: *TypeExprList = te.funcParams;
+        while cur != null as *TypeExprList {
+            let node: *TypeExprList = bux_alloc(sizeof(TypeExprList)) as *TypeExprList;
+            node.te = Lcx_SubstituteType(ctx, cur.te);
+            node.next = null as *TypeExprList;
+            if head == null as *TypeExprList {
+                head = node;
+                tail = node;
+            } else {
+                tail.next = node;
+                tail = node;
+            }
+            cur = cur.next;
+        }
+        r.funcParams = head;
+        r.typeName = Lcx_BuildFuncTypeName(r);
+        return r;
+    }
+
     return te;
 }