feat: capture-less anonymous functions (closures)

Implement MVP closures — anonymous functions without captures.

Syntax:
  |a: int, b: int| -> int { return a + b; }

Changes:
- ast.bux + ast.nim: add ekClosure AST node
- parser.bux + parser.nim: parse |params| -> Ret { body }
- sema.bux + sema.nim: type-check closure params/body, return tyFunc
- hir_lower.bux + hir_lower.nim: generate __closure_N function + hAddrOf
- lir_c_backend.nim: fix function-pointer variable declaration (cParamDecl)
- C backend: closures compile to global functions with unique names

Test: _test_closure/src/Main.bux
- Closure as variable
- Closure passed to higher-order function
- Address of named function as function pointer

Both bootstrap and selfhost compilers build and pass the test.

_test_closure/build/main.c

@@ -0,0 +1,67 @@
+// Generated by Bux C Backend v2
+#include <stdint.h>
+#include <stdbool.h>
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef const char* String;
+typedef unsigned char uint8;
+typedef unsigned short uint16;
+typedef unsigned int uint32;
+typedef unsigned long long uint64;
+typedef signed char int8;
+typedef short int16;
+typedef long long int64;
+typedef float float32;
+typedef double float64;
+typedef char char8;
+
+void* bux_alloc(unsigned int size);
+void bux_free(void* ptr);
+
+
+int Apply(int x, int (*op)(int));
+int Double(int x);
+int __closure_2(int a, int b);
+int __closure_3(int x);
+int main();
+
+void PrintInt(int x);
+void PrintLine(String msg);
+
+int Apply(int x, int (*op)(int)) {
+    return op(x);
+
+}
+
+int Double(int x) {
+    return x * 2;
+
+}
+
+int __closure_2(int a, int b) {
+    return a + b;
+
+}
+
+int __closure_3(int x) {
+    return x * 3;
+
+}
+
+int main() {
+    int (*add)(int, int) = &__closure_2;
+    int sum = add(3, 4);
+    PrintInt(sum);
+    PrintLine("");
+    int result = Apply(5, &__closure_3);
+    PrintInt(result);
+    PrintLine("");
+    int (*d)(int) = &Double;
+    PrintInt(d(7));
+    PrintLine("");
+    return 0;
+
+}
+