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/io.c

@@ -0,0 +1,54 @@
+/* Bux Standard Library - I/O functions */
+
+#include <stdio.h>
+#include <stdint.h>
+#include <string.h>
+
+/* PrintLine - print string with newline */
+void PrintLine(const char* s) {
+    if (s != NULL) {
+        puts(s);
+    } else {
+        puts("");
+    }
+}
+
+/* Print - print string without newline */
+void Print(const char* s) {
+    if (s != NULL) {
+        printf("%s", s);
+    }
+}
+
+/* PrintInt - print integer */
+void PrintInt(int n) {
+    printf("%d", n);
+}
+
+/* PrintInt64 - print 64-bit integer */
+void PrintInt64(int64_t n) {
+    printf("%lld", (long long)n);
+}
+
+/* PrintFloat - print float */
+void PrintFloat(double f) {
+    printf("%g", f);
+}
+
+/* PrintBool - print boolean */
+void PrintBool(int b) {
+    printf("%s", b ? "true" : "false");
+}
+
+/* ReadLine - read line from stdin (simplified) */
+const char* ReadLine(void) {
+    static char buffer[1024];
+    if (fgets(buffer, sizeof(buffer), stdin) != NULL) {
+        size_t len = strlen(buffer);
+        if (len > 0 && buffer[len-1] == '\n') {
+            buffer[len-1] = '\0';
+        }
+        return buffer;
+    }
+    return "";
+}